extract

(PHP 4, PHP 5, PHP 7, PHP 8)

extract — Importar variables a la tabla de símbolos actual desde un array

Descripción

extract(array &$array, int $flags = EXTR_OVERWRITE, string $prefix = null): int

Importa variables desde un array a la tabla de símbolos actual.

Comprueba cada clave para ver si tiene un nombre de variable válido. También comprueba las colisiones con las variables existentes en la tabla de símbolos.

Advertencia

No use extract() sobre datos no confiables, como la entrada del usuario (p.e. $_GET, $_FILES).

Parámetros

array

Un array asociativo. Esta función trata a las claves como nombres de variables y a los valores como valores de variables. Por cada par clave/valor creará una variable en la tabla de símbolos actual, sujeta a los parámetros flags y prefix.

Se debe usar un array asociativo; un array indexado numéricamente no producirá resultados a menos que se use EXTR_PREFIX_ALL o EXTR_PREFIX_INVALID.

flags

La manera en que las claves no-válidas/numéricas y las colisiones son tratadas está determinda por el parámetro flags. Puede ser uno de los valores siguientes:

EXTR_OVERWRITE: Si hay una colisión, sobrescribe la variable existente.
EXTR_SKIP: Si hay una colisión, no sobrescribe la variable existente.
EXTR_PREFIX_SAME: Si hay una colisión, prefija el nombre de la variable con prefix.
EXTR_PREFIX_ALL: Prefija todos los nombres de variables con prefix.
EXTR_PREFIX_INVALID: Sólo prefija los nombres de variables no-válidas/numéricas con prefix.
EXTR_IF_EXISTS: Sobrescribe la variable sólo si ya existe en la tabla de símbolos actual, de otro modo no hace nada. Esto es útil para definir una lista de variables válidas y después extraer sólo aquellas variables que se definieron fuera de $_REQUEST, por ejemplo.
EXTR_PREFIX_IF_EXISTS: Crea nombres de variables con prefijo sólo si la versión no prefijada de la misma variable existe en la tabla de símbolos actual.
EXTR_REFS: Extrae las variables como referencias. Esto significa que los valores de las variables importadas están todavía referenciados por los valores del parámetro array. Se puede usar esta bandera por sí misma o combinarla con cualquier otra bandera usando el operador OR en flags.

Si no se especifica flags, se asume que es EXTR_OVERWRITE.

prefix

Observe que prefix sólo es requerido si flags es EXTR_PREFIX_SAME, EXTR_PREFIX_ALL, EXTR_PREFIX_INVALID o EXTR_PREFIX_IF_EXISTS. Si el resultado prefijado no es un nombre de variable válido, ésta no es importada a la tabla de símbolos actual. Los prefijos son separados de las claves del array mediante un caracter de subrayado.

Valores devueltos

Devuelve el número de variables importadas con éxito a la tabla de símbolos.

Ejemplos

Ejemplo #1 Ejemplo de extract()

Un posible uso de extract() es importar a la tabla de símbolos las variables contenidas en un array asociativo devuelto por wddx_deserialize().

<?php

/* Se supone que $var_array es un array devuelto desde
   wddx_deserialize */

$tamaño = "grande";
$var_array = array("color" => "azul",
                   "tamaño"  => "medio",
                   "forma" => "esfera");
extract($var_array, EXTR_PREFIX_SAME, "wddx");

echo "$color, $tamaño, $forma, $wddx_tamaño\n";

?>

El resultado del ejemplo sería:

azul, grande, esfera, medio

El $tamaño no se ha sobrescrito ya que se especificó EXTR_PREFIX_SAME, lo que resulta en la creación de $wddx_tamaño. Si se hubiera especificado EXTR_SKIP, $wddx_tamaño no se habría creado. EXTR_OVERWRITE causaría que $tamaño tuviera el valor "medio", y EXTR_PREFIX_ALL resultaría en variables nuevas llamadas $wddx_color, $wddx_tamaño, y $wddx_forma.

Notas

Advertencia

No use extract() en datos que no son de confianza, como las entradas de usuario (esto es, $_GET, $_FILES, etc.). Si lo hace, asegúrese de usar uno de los valores de flags que no sobrescriban, como EXTR_SKIP, y sepa que debería realizar la extracción en el mismo orden que fue definido en variables_order dentro de php.ini.

Ver también

compact() - Crear un array que contiene variables y sus valores
list() - Asignar variables como si fueran un array

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User Contributed Notes 28 notes

down

Michael Newton ¶

19 years ago

They say "If the result is not a valid variable name, it is not imported into the symbol table."

What they should say is that if _any_ of the results have invalid names, _none_ of the variables get extracted.

Under 4.3.10 on Windows 2000, I was pulling some mySQL records, but needed to convert two fields into IP addresses:
<?
extract(mysql_fetch_assoc(mysql_query('SELECT * FROM foo')));
extract(mysql_fetch_assoc(mysql_query('SELECT INET_NTOA(bar) AS bar, INET_NTOA(baz) FROM foo')));
?>

I had forgotten the second AS modifier in the SQL query.  Because it couldn't extract a variable called INET_NTOA(baz) into the symbol table, it didn't do either of them.

(BTW I don't normally stack functions up like that!  Just to make a short example!)

down

FredLawl ¶

11 years ago

It is possible to use this as a way to create public attributes for a class.

<?php
class Foo {

  public function __construct ($array) {

      extract($array, EXTR_REFS);
      foreach ($array as $key => $value) {
          $this->$key = $$key;
          // Do: $this->key = $key; if $key is not a string.
      }

  }

}

$array = array(
    'valueOne'      => 'Test Value 1',
    'valueTwo'      => 'Test Value 2',
    'valueThree'    => 'Test Value 3'
);

$foo = new Foo($array);

// Works
echo $foo->valueOne; // Test Value 1
echo $foo->valueTwo; // Test Value 2

// Does not work!
echo $foo::$valueOne; // Fatal error:  Access to undeclared static property: Test::$valueOne
?>

down

Csaba at alum dot mit dot edu ¶

18 years ago

Sometimes you may want to extract only a named subset of the key/value pairs in an array.  This keeps things more orderly and could prevent an unrelated variable from getting clobbered from an errant key.  For example,

$things = 'unsaid';
$REQUEST = array(He=>This, said=>1, my=>is, info=>2, had=>a,
                 very=>3, important=>test, things=>4);
$aVarToExtract = array(my, important, info);
extract (array_intersect_key ($REQUEST, array_flip($aVarToExtract)));

will extract
$my = 'is';
$important = 'test';
$info = 2;

but will leave certain
$things = 'unsaid'

Csaba Gabor from Vienna
NB.  Of course the composite request coming in from a web page is in $_REQUEST.

down

dmikam ¶

10 years ago

I have made some tests to compare the speed of next constructions:
<?php

extract($ARRAY);

// vs. 

foreach($ARRAY as $key=>$value) 
    $$key = $value; 
?>

Surprisingly for me extract is 20%-80% slower then foreach construction. I don't really understand why, but it's so.

down

Robc ¶

13 years ago

When extracting from a row after a database query using for example:

$row = mysql_fetch_array($result, MYSQL_ASSOC)
extract($row);

I find that the resultant variables may not match the variable type in the database.  In particular I have found integers in the database may gettype() to string on the extracted variable.

down

Dan O'Donnell ¶

17 years ago

Following up on ktwombley at gmail dot com's post:

Presumably one easy way of dealing with this security issue is to use the EXTR_IF_EXISTS flag and make sure 

a) your define acceptable input variables beforehand (i.e. as empty variables)
b) Sanitise any user input to avoid unacceptable variable content. 

If you do these two things, then I'm not sure I see the difference between extract($_REQUEST,EXTR_IF_EXISTS); and assigning each of the variables by hand.

I'm not talking here about the idea of storing the variables in a database, just the immediately necessary steps to allow you to use extract on REQUEST arrays with relative safety.

down

CertaiN ¶

10 years ago

[New Version]

Example Usage:
<?php
$_GET['A']['a'] = '  CORRECT(including some spaces)    ';
$_GET['A']['b'] = '  CORRECT(including some spaces)    ';
$_GET['A']['c'] = "Invalid UTF-8 sequence: \xe3\xe3\xe3";
$_GET['A']['d']['invalid_structure'] = 'INVALID';

$_GET['B']['a'] = '  CORRECT(including some spaces)    ';
$_GET['B']['b'] = "Invalid UTF-8 sequence: \xe3\xe3\xe3";
$_GET['B']['c']['invalid_structure'] = 'INVALID';
$_GET['B']["Invalid UTF-8 sequence: \xe3\xe3\xe3"] = 'INVALID';

$_GET['C']['a'] = '  CORRECT(including some spaces)    ';
$_GET['C']['b'] = "Invalid UTF-8 sequence: \xe3\xe3\xe3";
$_GET['C']['c']['invalid_structure'] = 'INVALID';
$_GET['C']["Invalid UTF-8 sequence: \xe3\xe3\xe3"] = 'INVALID';

$_GET['unneeded_item'] = 'UNNEEDED';

var_dump(filter_struct_utf8(INPUT_GET, array(
    'A' => array(
        'a' => '',
        'b' => FILTER_STRUCT_TRIM,
        'c' => '',
        'd' => '',
    ),
    'B' => FILTER_STRUCT_FORCE_ARRAY,
    'C' => FILTER_STRUCT_FORCE_ARRAY | FILTER_STRUCT_TRIM,
)));
?>

Example Result:
array(3) {
  ["A"]=>
  array(4) {
    ["a"]=>
    string(36) "  CORRECT(including some spaces)    "
    ["b"]=>
    string(30) "CORRECT(including some spaces)"
    ["c"]=>
    string(0) ""
    ["d"]=>
    string(0) ""
  }
  ["B"]=>
  array(3) {
    ["a"]=>
    string(36) "  CORRECT(including some spaces)    "
    ["b"]=>
    string(0) ""
    ["c"]=>
    string(0) ""
  }
  ["C"]=>
  array(3) {
    ["a"]=>
    string(30) "CORRECT(including some spaces)"
    ["b"]=>
    string(0) ""
    ["c"]=>
    string(0) ""
  }
}

down

amolocaleb at gmail dot com ¶

6 years ago

If an object is typecasted into an array and "extracted",only the public properties will be accessible.Methods are of course omitted.
<?php
class Test{
    public $name = '';

    protected $age = 10;

    public $status = 'disabled';

    private $isTrue = false;

    public function __construct()
    {
        $this->name = 'Amolo';
        $this->status = 'active';
    }

    public function getName()
    {
        return $this->name;
    }

    public function getAge()
    {
        return $this->age;
    }

    public function getStatus()
    {
        return $this->status;
    }

}

$obj = (array) new Test();
var_dump($obj);
/* array(4) { ["name"]=> string(5) "Amolo" ["*age"]=> int(10) ["status"]=> string(6) "active" ["TestisTrue"]=> bool(false) } */
extract((array)new Test());
echo  $name; //Amolo
echo  $status; //active
echo $age;//Notice: Undefined variable: age
echo $isTrue;//Notice: Undefined variable: isTrue

down

phatsk+php at gmail dot com ¶

6 years ago

Using extract's return parameter can lead to unintended results, particularly with EXTR_REFS:

<?php

$my_data = [
    'count' => 15,
    'name' => 'foo',
];

$count = extract( $my_data, EXTR_REFS );

echo $my_data['count']; // 2, not 15.

down

dotslash.lu at gmail.com ¶

11 years ago

You can't extract a numeric indexed array(e.g. non-assoc array).
<?php
$a = array(
  1,
  2
);
extract($a);
var_dump(${1});
?>

result:
PHP Notice:  Undefined variable: 1 in /Users/Lutashi/t.php on line 7

Notice: Undefined variable: 1 in /Users/Lutashi/t.php on line 7
NULL

down

mrkhoa99 at gmail dot com ¶

6 years ago

We can use extract () function for Template Engine:

<?php
#Template.php

class Template
{
    protected $viewVars;

    public function renderPage($tpl)
    {
        ob_start();
        extract($this->viewVars, EXTR_SKIP);
        include $tpl;
        return ob_end_flush();
    }

    public function assign($arr)
    {
        foreach ($arr as $key => $value) {
            $this->viewVars[$key] = $value;
        }
        return $this;
    }
}

$template = new Template();
$template->assign(
   [ 'pageHeader' => 'Page Header', 'content' => 'This is the content page']
);
$template->renderPage('tpl.php');

#tpl.php

<h1><?= $pageHeader; ?></h1>
<p><?= $content ;?></p>

Output:

Page Header
This is the content page

down

CertaiN ¶

10 years ago

[New Version]
This function is very useful for filtering complicated array structure.
Also, Some integer bitmasks and invalid UTF-8 sequence detection are available.

Code:
<?php
/**
 * @param  integer $type    Constant like INPUT_XXX.
 * @param  array   $default Default structure of the specified super global var.
 *                          Following bitmasks are available:
 *  + FILTER_STRUCT_FORCE_ARRAY - Force 1 dimensional array.
 *  + FILTER_STRUCT_TRIM        - Trim by ASCII control chars.
 *  + FILTER_STRUCT_FULL_TRIM   - Trim by ASCII control chars,
 *                                full-width and no-break space.
 * @return array            The value of the filtered super global var.
 */
define('FILTER_STRUCT_FORCE_ARRAY', 1);
define('FILTER_STRUCT_TRIM', 2);
define('FILTER_STRUCT_FULL_TRIM', 4);
function filter_struct_utf8($type, array $default) {
    static $func = __FUNCTION__;
    static $trim = "[\\x0-\x20\x7f]";
    static $ftrim = "[\\x0-\x20\x7f\xc2\xa0\xe3\x80\x80]";
    static $recursive_static = false;
    if (!$recursive = $recursive_static) {
        $types = array(
            INPUT_GET => $_GET,
            INPUT_POST => $_POST,
            INPUT_COOKIE => $_COOKIE,
            INPUT_REQUEST => $_REQUEST,
        );
        if (!isset($types[(int)$type])) {
            throw new LogicException('unknown super global var type');
        }
        $var = $types[(int)$type];
        $recursive_static = true;
    } else {
        $var = $type;
    }
    $ret = array();
    foreach ($default as $key => $value) {
        if ($is_int = is_int($value)) {
            if (!($value | (
                FILTER_STRUCT_FORCE_ARRAY |
                FILTER_STRUCT_FULL_TRIM | 
                FILTER_STRUCT_TRIM
            ))) {
                $recursive_static = false;
                throw new LogicException('unknown bitmask');
            }
            if ($value & FILTER_STRUCT_FORCE_ARRAY) {
                $tmp = array();
                if (isset($var[$key])) {
                    foreach ((array)$var[$key] as $k => $v) {
                        if (!preg_match('//u', $k)){
                            continue;
                        }
                        $value &= FILTER_STRUCT_FULL_TRIM | FILTER_STRUCT_TRIM;
                        $tmp += array($k => $value ? $value : '');
                    }
                }
                $value = $tmp;
            }
        }
        if ($isset = isset($var[$key]) and is_array($value)) {
            $ret[$key] = $func($var[$key], $value);
        } elseif (!$isset || is_array($var[$key])) {
            $ret[$key] = null;
        } elseif ($is_int && $value & FILTER_STRUCT_FULL_TRIM) {
            $ret[$key] = preg_replace("/\A{$ftrim}++|{$ftrim}++\z/u", '', $var[$key]);
        } elseif ($is_int && $value & FILTER_STRUCT_TRIM) {
            $ret[$key] = preg_replace("/\A{$trim}++|{$trim}++\z/u", '', $var[$key]);
        } else {
            $ret[$key] = preg_replace('//u', '', $var[$key]);
        }
        if ($ret[$key] === null) {
            $ret[$key] = $is_int ? '' : $value;
        }
    }
    if (!$recursive) {
        $recursive_static = false;
    }
    return $ret;
}
?>

down

nicolas zeh ¶

18 years ago

This function provides exactly the same functionality as extract except that a parameter was added defining the extract target.
This function can be used if your PHP installation does not support the required Flags or more important if you would like to extract arrays to another destination as to $GLOBALS, i.e. other arrays or objects.
The only difference to extract is that extract_to moves the array pointer of $arr to the end as $arr is passed by reference to support the EXTR_REFS flag.

<?php
    if( !defined('EXTR_PREFIX_ALL') ) define('EXTR_PREFIX_ALL', 3);
    if( !defined('EXTR_PREFIX_INVALID') ) define('EXTR_PREFIX_INVALID', 4);
    if( !defined('EXTR_IF_EXISTS') ) define('EXTR_IF_EXISTS', 5);
    if( !defined('EXTR_PREFIX_IF_EXISTS') ) define('EXTR_PREFIX_IF_EXISTS', 6);
    if( !defined('EXTR_REFS') ) define('EXTR_REFS', 256);
    
    
    function extract_to( &$arr, &$to, $type=EXTR_OVERWRITE, $prefix=false ){
        
        if( !is_array( $arr ) ) return trigger_error("extract_to(): First argument should be an array", E_USER_WARNING );
        
        if( is_array( $to ) ) $t=0;
        else if( is_object( $to ) ) $t=1;
        else return trigger_error("extract_to(): Second argument should be an array or object", E_USER_WARNING );
        
        if( $type==EXTR_PREFIX_SAME || $type==EXTR_PREFIX_ALL || $type==EXTR_PREFIX_INVALID || $type==EXTR_PREFIX_IF_EXISTS )
            if( $prefix===false ) return trigger_error("extract_to(): Prefix expected to be specified", E_USER_WARNING ); 
            else $prefix .= '_';
        
        $i=0;
        foreach( $arr as $key=>$val ){
            
            $nkey = $key;
            $isset = $t==1 ? isset( $to[$key] ) : isset( $to->$key );
            
            if( ( $type==EXTR_SKIP && $isset ) 
                || ( $type==EXTR_IF_EXISTS && !$isset ) )
                    continue;
            
            else if( ( $type==EXTR_PREFIX_SAME && $isset )
                || ( $type==EXTR_PREFIX_ALL )
                || ( $type==EXTR_PREFIX_INVALID && !preg_match( '#^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$#', $key ) ) )
                    $nkey = $prefix.$key;
                    
            else if( $type==EXTR_PREFIX_IF_EXISTS )
                if( $isset ) $nkey = $prefix.$key;
                else continue;

            if( !preg_match( '#^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$#', $nkey ) ) continue;
            
            if( $t==1 )
                if( $type & EXTR_REFS ) $to->$nkey = &$arr[$key];
                else $to->$nkey = $val;
            else 
                if( $type & EXTR_REFS ) $to[$nkey] = &$arr[$key];
                else $to[$nkey] = $val;
            
            $i++;
        }
        
        return $i;
    }
    
    // e.g.:
    extract_to( $myarray, $myobject, EXTR_IF_EXISTS );
?>

down

benjaminATwebbutvecklarnaDOTse ¶

16 years ago

Re: anon at anon dot org, about extract() and null values

Personally I've found use extracting multiple resultsets from db where the latter would overwrite the previous when a variable is not null ( and optionally if its not >0 )

It would be useful if $extract_type was extended on top of these two:

EXTR_OVERWRITE
EXTR_SKIP

with something like this:

EXTR_OVERWRITE_NULL 
 - If there is a collision, overwrite the existing variable if it is null 

EXTR_OVERWRITE_0
 - Same thing but == 0 or null

EXTR_SKIP_NULL
 - If there is a collision, skip the new variable if the existing is not null

EXTR_SKIP_0
 - Same thing but == 0 or null

Those ought to cover a few good cases that aren't covered now.

down

bob ¶

9 years ago

Note that extract() will only create or overwrite variables in the current scope, so
<?
function test(){
 $a=Array('b'=>1,'c'=>2);
 extract($a);
}
test();
exit("$b");
?>
will produce no output, whereas
<?
function test(){
 global $b;
 $a=Array('b'=>1,'c'=>2);
 extract($a);
}
test();
exit("$b");
?>
will output 1.

down

Hayley Watson ¶

16 years ago

Dan O'Donnell's suggestion needs a third requirement to work as described:

c) No other variables are defined - especially variables that contain potentially sensitive information.

Without that condition the difference between extract() and assigning variables by hand (and the resulting security implications) should be obvious.

The only valid security step there is (b) - but you should be doing that anyway.

down

owk dot ch199_ph at gadz dot org ¶

18 years ago

And if you want with PHP 5 an easy way to extract $V by reference, try this :
<?php
    foreach ($V as $k => &$v) {
        $$k =& $v;
    }
?>
It can be used to create special kind of "free args" functions that let you choose when you call them the way you send variables, and which ones. They are moreover very fast to call thanks to references :
<?php
    function free_args (&$V) {
        foreach ($V as $k => &$v) {
            $$k =& $v;
        }
        unset ($k);  unset ($v);  unset ($V);

        // be careful that if you need to extract $k, $v or $V variables you should find other names for them in the lines above (ie. $__k, $__v and $__V)
    }

    $huge_text = '...';

    $a = array ('arg1' => 'val1', 'arg2' => &$huge_text); // in this call, only $arg2 will be a true reference in the function
    free_args ($a);
?>
Be warned that you can't write : "<?php free_args (array ('arg1' => 'val1')); ?>" because the array can't be referenced by the function, as it's not yet created when the function starts.

down

ktwombley at gmail dot com ¶

18 years ago

It's really easy to open gaping security holes using extract() on $_REQUEST, $_GET, etc. You have to be really sure of what you're doing, and use the proper flags on extract() to avoid clobbering important variables.

For instance, the submission by kake26 at gmail dot com will not only perfectly emulate register globals (that's bad), but it'll store it in a database and recall the same variables every time the script runs (essentially allowing an attacker to attack your script every time it runs via one attack). Oops!

To fix it, you'd have to get creative with flags. Maybe you could use EXTR_PREFIX_ALL instead of EXTR_OVERWRITE, for example. Of course, you should also sanitize the form elements to ensure there's no php code in them, and also to make sure any very important variables aren't in the form data. (like the classic $is_admin = true attack)

down

Adam Monsen <adamm at wazamatta dot com> ¶

20 years ago

As shown in the example, if your 'prefix' is used, a single underscore is added to the name of the extracted variable. Meaning, a prefix of 'p' becomes a prefix of 'p_', so 'blarg' prefixed would be 'p_blarg'.

If you're not sure what variables you've created through extraction, you can call get_defined_vars() to see all defined variables in the current scope.

down

Anonymous ¶

19 years ago

To make this perfectly clear (hopefully), an underscore is always added when the string is prefixed.
extract(array("color" => "blue"),EXTR_PREFIX_ALL,'');// note: prefix is empty
is the same as
$color='_blue';

down

Dutchdavey ¶

17 years ago

I would draw your attention to the user note at the very end of this page regarding PREFIXES. The user points out that php adds a '_' to your prefixes.

down

anon at anon dot org ¶

19 years ago

A warning about extract() and null values.

This might be an actual Zend2 Engine bug, but it's bad programming practice, so I'm sharing it here instead.

I often work in envrionments where E_STRICT (which would prevent errors like this) isn't on, and I don't have access to change it. I also use a very simple template class that in a nutshell works like this:

$t = new Template('somefile.php');
$t->title = $title;
$t->body = $body;
$t->display();

display() more or less looks like this:

function display(){
    extract(get_object_vars($this),EXTR_REFS);
    ob_start(); include $this->templateFileName;
    return ob_get_clean();
}

If any of the assigned values are null (let's say that in this case $title wasn't initialized above) it causes the engine to do all sorts of incredibly whacky stuff like certifiably lose track of variables in an incredibly inconsistent way. I traced the problem down to the fact that it's using the EXTR_REFS flag. I assume that in PHP's internal variable storage or reference counting mechanism, that trying to extract null references makes it lose track or count of something or rather.

In a nutshell, if you start getting wierd behavior when using extract() make sure that the array or object you are trying to get variables out of doesn't contain null keys or values!

down

auto493097 at hushmail dot com ¶

14 years ago

I use XDebug with NetbeansIDE to for analyzing and developing PHP Code. When debugging an extract statement no new variables appeared in the variable's list. Although all variables created by extract could be examined by explicit watch items and single variables appeared as soon as an PHP script makes use of them I am not sure weather it is a wrong configuration, a feature or a bug in XDebug.

down

Aaron Stone ¶

20 years ago

If you are working porting an older application, and taking the advice above, extracting only _SERVER, _SESSING, _COOKIE, _POST, _GET, you have forgotten to extract _FILES. Putting _FILES last and using EXTR_SKIP doesn't work because the name of the file upload box is already set as a variable containing only the temporary name of the uploaded file from one of the earlier extracts (I haven't tested to see which one specifically, however). A workaround is to put _FILES last and use EXTR_OVERWRITE. This allows extract to replace that temp-name-only variable with the full array of file upload information.

down

-1

darrenforster99 at gmail dot com ¶

3 years ago

In response to Dan O'Donnell's note:

"Presumably one easy way of dealing with this security issue is to use the EXTR_IF_EXISTS flag and make sure"

Not necessarily - even using the EXTR_IF_EXISTS flag could be extremely dangerous - imagine this code running...

<?php

global $sql ;

function runSql ()
{
  $result = $conn->query($sql);
  $conn-> close();
  return $result;
}

function extractGet ()
{
  $name = '' ;
  $address = '' ;
  foreach ( $_GET as $key => $value )
    $_GET[$key] = urldecode ( $value ) ;
  extract($_GET,EXTR_IF_EXISTS);
  $sql = str_replace ( '{NAME}', $name, $sql ) ;
  $sql = str_replace ( '{ADDRESS}', $address, $sql ) ;
}

function outputResult ( $res )
{
  echo '<pre>'.print_r ( $res->fetch_array(MYSQLI_NUM) ).'</pre>' ;
}

$sql = 'SELECT postcode FROM Customers WHERE name={NAME} AND address={ADDRESS}';
extractGet () ;
$res = runSql () ;
outputResult ( $res ) ;
?>

Now can you see a massive security issue here...

seems all well and good if we had a url like 

mycode.php?name=joe%20bloggs&address=20%20Any%20Street

as that would specifically find joe bloggs of 20 any street.

however what if someone typed in

mycode.php?sql=SELECT%20password%20FROM%20Customers

or even worse

mycode.php?sql=DELETE%20from%20Customers

The problem here is that even though as far as you're aware you defined both name and address as the only two empty variables within that function - you may have forgotten about global variables, and these global variables could cause major security issues

In the example above $sql is defined as "SELECT postcode FROM Customers WHERE name={NAME} AND address={ADDRESS}" which seems all well and good, and safe, and then later on str_replace in the extractGet function replaces {NAME} and {ADDRESS} with the name and address variables from $_GET - but if GET contains an SQL variable then that would overwrite the global $sql variable before the str_replace function - and if the str_replace function finds no matches it just returns the original string - in the above two examples the SQL string would be "SELECT password FROM Customers" which in the example outputResult just prints the data retrieved from the database and so in this stage it could print all the customers passwords (hopefully encrypted!) to the screen (oops!) or in the second example the SQL string would be "DELETE from Customers" - with no WHERE clause that would delete all the data from the Customers table and of course a combination of

SELECT table_name FROM information_schema.tables

and

DROP TABLE <table_name>

could be a real disaster!

Of course, this is only a basic example, but it could be quite easy to forget about global variables and these global variables could quite easily be used with extract to cause serious security risks if GET, REQUEST or POST is sent to extract!

down

-2

pg dot perfection at gmail dot com ¶

19 years ago

Here is a little example of how an extraction method should look like when it needs to work recursive (work on nested_arrays too)...

Note that this is only an example, it can be done more easily, and more advanced too.

<?php
/**
 * A nested version of the extract () function.
 *
 * @param    array    $array        The array which to extract the variables from
 * @param    int        $type        The type to use to overwrite (follows the same as extract () on PHP 5.0.3
 * @param    string    $prefix        The prefix to be used for a variable when necessary
 */
function extract_nested (&$array, $type = EXTR_OVERWRITE, $prefix = '')
{
    /**
     *  Is the array really an array?
     */
    if (!is_array ($array))
    {
        return trigger_error ('extract_nested (): First argument should be an array', E_USER_WARNING);
    }

    /**
     *  If the prefix is set, check if the prefix matches an acceptable regex pattern 
     * (the one used for variables)
     */
    if (!empty ($prefix) && !preg_match ('#^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$#', $prefix))
    {
        return trigger_error ('extract_nested (): Third argument should start with a letter or an underscore', E_USER_WARNING);
    }

    /**
     * Check if a prefix is necessary. If so and it is empty return an error.
     */
    if (($type == EXTR_PREFIX_SAME || $type == EXTR_PREFIX_ALL || $type == EXTR_PREFIX_IF_EXISTS) && empty ($prefix))
    {
        return trigger_error ('extract_nested (): Prefix expected to be specified', E_USER_WARNING);
    }

    /**
     * Make sure the prefix is oke
     */
    $prefix = $prefix . '_';

    /**
     *  Loop thru the array
     */
    foreach ($array as $key => $val)
    {
        /**
         *  If the key isn't an array extract it as we need to do
         */
        if (!is_array ($array[$key]))
        {
            switch ($type)
            {
                default:
                case EXTR_OVERWRITE:
                    $GLOBALS[$key] = $val;
                break;
                case EXTR_SKIP:
                    $GLOBALS[$key] = isset ($GLOBALS[$key]) ? $GLOBALS[$key] : $val;
                break;
                case EXTR_PREFIX_SAME:
                    if (isset ($GLOBALS[$key]))
                    {
                        $GLOBALS[$prefix . $key] = $val;
                    }
                    else
                    {
                        $GLOBALS[$key] = $val;
                    }
                break;
                case EXTR_PREFIX_ALL:
                    $GLOBALS[$prefix . $key] = $val;
                break;
                case EXTR_PREFIX_INVALID:
                    if (!preg_match ('#^[a-zA-Z_\x7f-\xff]$#', $key{0}))
                    {
                        $GLOBALS[$prefix . $key] = $val;
                    }
                    else
                    {
                        $GLOBALS[$key] = $val;
                    }
                break;
                case EXTR_IF_EXISTS:
                    if (isset ($GLOBALS[$key]))
                    {
                        $GLOBALS[$key] = $val;
                    }
                break;
                case EXTR_PREFIX_IF_EXISTS:
                    if (isset ($GLOBALS[$key]))
                    {
                        $GLOBALS[$prefix . $key] = $val;
                    }
                break;
                case EXTR_REFS:
                    $GLOBALS[$key] =& $array[$key];
                break;
            }
        }
        /**
         *  The key is an array... use the function on that index
         */
        else
        {
            extract_nested ($array[$key], $type, $prefix);
        }
    }
}
?>

down

-4

danbettles at yahoo dot co dot uk ¶

15 years ago

When using EXTR_PREFIX_ALL - and probably all the other EXTR_PREFIX_* constants - and a numerically-indexed array, extract() will add an underscore ("_") between the prefix and the index.

<?php

extract(array('foo', 'bar'), EXTR_PREFIX_ALL, 'var');

print_r(get_defined_vars());  // Reveals $var_0 = 'foo' and $var_1 = 'bar'
?>

down

-5

moslehi<atsign>gmail<d0t>c0m ¶

18 years ago

Experimentally I found that calling extract() also shows the number of keys if the key is set and is not numeric ! Maybe there was a better definition than mine  . Please have a look to this scripts :

<?PHP 
$var["i"] = "a";
$var["j"] = "b";
$var["k"] = 1;
echo extract($var); // returns 3
?>

<?PHP 
$var2["i"] = "a";
$var2[2] = "b";
$var2[] = 1;
echo extract($var2); // returns 1
?>

(Arash Moslehi)

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