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    User Contributed Notes 19 notes

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    22
    olszewski_marek at yahoo dot com
    23 years ago
    Here is a simple way to control who downloads your files...

    You will have to set: $filename, $downloaddir, $safedir and $downloadURL.

    Basically $filename is the name of a file, $downloaddir is any dir on your server, $safedir is a dir that is not accessible by a browser that contains a file named $filename and $downloadURL is the URL equivalent of your $downloaddir.

    The way this works is when a user wants to download a file, a randomly named dir is created in the $downloaddir, and a symbolic link is created to the file being requested. The browser is then redirected to the new link and the download begins.

    The code also deletes any past symbolic links created by any past users before creating one for itself. This in effect leaves only one symbolic link at a time and prevents past users from downloading the file again without going through this script. There appears to be no problem if a symbolic link is deleted while another person is downloading from that link.

    This is not too great if not many people download the file since the symbolic link will not be deleted until another person downloads the same file.

    Anyway enjoy:

    <?php
    $letters     = 'abcdefghijklmnopqrstuvwxyz';
    srand((double) microtime() * 1000000);
    $string = '';
    for (    $i = 1; $i <= rand(4,12); $i++) {
    $q = rand(1,24);
    $string = $string . $letters[$q];
    }
    $handle = opendir($downloaddir);
    while (    $dir = readdir($handle)) {
    if (    is_dir($downloaddir . $dir)){
    if (    $dir != "." && $dir != ".."){
    @    unlink($downloaddir . $dir . "/" . $filename);
    @    rmdir($downloaddir . $dir);
    }
    }
    }
    closedir($handle);
    mkdir($downloaddir . $string, 0777);
    symlink($safedir . $filename, $downloaddir . $string . "/" . $filename);
    Header("Location: " . $downloadURL . $string . "/" . $filename);
    ?>
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    4
    Wixiweb
    6 years ago
    If you use the suhosin extension, you must allow symlink with :
    > suhosin.executor.allow_symlink = On

    Check your /var/log/syslog file for more information :
    > suhosin: ALERT - symlink called during open_basedir
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    10
    reddy1042001 at yahoo dot co dot in
    14 years ago
    Symlinks on windows are created by Symlink() which accept only absolute paths but not relative paths .relative paths on windows are not supported for symlinks
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    4
    Anonymous
    17 years ago
    Um, duh... that's all I gotta say about by previous note. Please delete it. :)

    Windows Vista has its own symlink program now (mklink). Hopefully future versions of PHP for Windows will have this function put it?

    Anyway, this will work on Vista (assuming your PHP user has the proper permissions):
    <?php
    define    ('SYMLINK_FILE', 0);
    define('SYMLINK_DIR', 1);
    define('SYMLINK_JUNCTION', 2);
    function     symlink ($target, $link, $flag = SYMLINK_FILE) {
    switch (    $flag) {
    case     SYMLINK_DIR: $pswitch = '/d'; break;
    case     SYMLINK_JUNCTION: $pswitch = '/j'; break;
    case     SYMLINK_FILE:
    default:     $pswitch = ''; break;
    }
    // Change / to \ because it will break otherwise.
    $target = str_replace('/', '\\', $target);
    $link = str_replace('/', '\\', $link);
    return     exec('mklink ' . $pswitch . ' "' . $link . '" "' . $target . '"');
    }
    ?>
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    2
    Numien
    17 years ago
    The one difference to using symlinks for controlled file access vs. readfile() is that the HTTP server will handle content-type of the symlink automatically.

    If you always want it to be downloaded, this can be a negative point. However, if you want a file of non-predefined type to be viewable in the browser, this can be a real asset.

    Of course, you can use fileinfo/mime-magic to do that, but those require a module which isn't always available on shared hosting.
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    5
    zeomniscient
    10 years ago
    Context: php cli on windows OS.

    Do not forget to start the console with "Run as Administrator" else symlink will return 'false' and raise the following error :
    Warning: symlink(): Cannot create symlink, error code(1314)
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    2
    flobee
    9 years ago
    sym/hard linking i love to do it relative a lot but not that trivial when thinking wrong :-)
    manpage says: the target is the _linkname_ and from that point to create relative links successful is to start from the directory where the link should be created e.g.:
    /myfiles/now/here/
    /myfiles/links/

    chdir( realpath(dirname($target)));
    You need to create/find out the relativ path to the source, otherwise the absolut path will be used in a link.

    if you start to create the target path from the source eg:
    "/myfiles/now/here/../../later/" and links should go to /myfiles/later/ you need to use realpath( dirname() ) set the new location using chdir() and start from here to link to the source which is ../now/here/
    This you can find out e.g.: when having the absolut path of source and target:
    $dirTo = realpath(dirname($to));
    chdir($dirTo);
    $linkName = basename($to);
    $target = $dirTo . DIRECTORY_SEPARATOR . $linkName;
    $srcFile = getRelativeDirectory($from, $to); //basicly a function which will find the from -> to, but in this case the from ist the target where you need to start.
    below you can find some examples how to split the path in chunks to find a relativ path
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    2
    anything at the domain williamfrantz.com
    18 years ago
    Olszewski_marek makes a good suggestion, but the readfile() function can also be used to obscure file downloads from end users.

    /* Setup the file that will be sent */
    $downloadDir = "some/secret/directory/";
    $file = "theFileName.dat";

    /* Required for IE, otherwise Content-disposition is ignored */
    if(ini_get('zlib.output_compression')) ini_set('zlib.output_compression', 'Off');

    /* Output HTTP headers that force "Save As" dialog */
    header("Pragma: public");
    header("Expires: 0");
    header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
    header("Cache-Control: private",false);
    header("Content-Type: application/octet-stream");
    header("Content-Disposition: attachment; filename=\\"$file\\";");
    header("Content-Transfer-Encoding: binary");
    header("Content-Length: ".@filesize($downloadDir.$file));

    /* Prevent the script from timing out for large files */
    set_time_limit(0);

    /* Send the entire file using @ to ignore all errors */
    @readfile($downloadDir.$file);

    /* Exit immediately so no garbage follows the file contents */
    exit;
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    1
    alvaro at demogracia dot com
    6 years ago
    To remove symlinks you need to use:

    - rmdir() on Windows
    - unlink() elsewhere

    <?php
    function removeSymlink($path) {
    if (    PHP_SHLIB_SUFFIX==='dll') {
    return     rmdir($path);
    }
    return     unlink($path);
    }
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    1
    raat1979 at gmail dot com
    9 years ago
    Be warned that at least with php 5.3 on windows 2008 R2 the symlink function is effected by the statcache.

    I got error 183 indicating the link existed. While the symlink was actually moved to a different location by another process and would certainly not exist anymore.
    Calling clearstatcache with a filename does not work. Most probably as the filename is converted to a device path (e.g. "\Device\HarddiskVolume1\D-Drive\www\pp\#static\index.html") which is a requirement for the windows call.

    easiest is to just call clearstatcache without a filename to resolve the issue.

    if you really want to call clearstatcache with a filename you could use readlink on the already deleted symlink (that still lives in the statcache) to get the filename.
    For example something like this:
    <?php
    if(@symlink('d:/www/pp/#static/index.html','d:/www/index.html')===false){
    //link failed, perhaps a statcache issue? We try to get the target, if it is a statcache issue this should work just fine

    if(($rl = @readlink($target))===false) return false; //not a cache issue as stat on a non-existing file failed, something else must be wrong...

    //clear the stat cache for the actual target, strangely enough this works even-though this is the target and not the symlink to be created (*)
    clearstatcache(true,$rl);

    if(@    symlink($source,$target) ===false) return false; //can't create the symlink
    ?>

    *It looks for PHP on windows like the statcache is maintained on the target instead of the actual symlink (this would ease in getting the file properties and limit the size of the statcache)
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    1
    Anonymous
    18 years ago
    <?

    /**
    * A function to emulate symbolic links on Windows.
    * Uses the junction utility available at:
    * http://www.sysinternals.com
    * Note that this will only work on NTFS volumes.
    *
    * The syntax of the junction utility is:
    * junction <junction directory> <junction target>
    *
    * Note that the parameter order of the Junction command
    * is the reverse of the symlink function!
    *
    * @param string $target
    * @param string $link
    */
    function _symlink( $target, $link ) {
    if ($_SERVER['WINDIR'] || $_SERVER['windir']) {
    exec('junction "' . $link . '" "' . $target . '"');
    } else {
    symlink($target,$link);
    }
    }

    function _unlink($link ) {
    if ($_SERVER['WINDIR'] || $_SERVER['windir']) {
    exec('junction -d "' . $link . '"');
    } else {
    unlink($link);
    }
    }
    ?>
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    1
    Calin
    12 years ago
    On IIS (Internet Information Services), you need to set permissions to allow the creation of symbolic links.

    Go to Local Security Policy -> Local Policies -> User Rights Assignment and right click on Create symbolic links -> Properties -> Add User or Group and add the "IUSR" user, which should be the user associated with IIS.
    A restart may be required.

    If this doesn't work, go to IIS Manager -> Authentication -> select Anonymous Authentication -> Edit and enter your current logged in user, which should be an administrator.
    Hit OK and now the symlink() function in php should work fine.
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    0
    php-user at protonmail dot com
    1 year ago
    Something that is not documented is that this function checks whether the target file exists. Such behavior can be quite inconvenient if you want to create several symbolic links to locally mounted remote files, for example with Rclone. In this specific case this function can take 2 or even more seconds to finish.
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    0
    Scare!
    2 years ago
    On Windows, symlink() does not accept paths with symbolic links in them. Use realpath() https://www.php.net/manual/en/function.realpath.php to expand the paths first.
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    0
    lingtalfi
    10 years ago
    If symlink seems not to work correctly, try to define the target argument as an absolute path.
    Coding on MacOsX PHP5.5, I noticed that in some cases using an absolute path on the target argument was the only way I could have a proper symlink created.
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    0
    ewering at gmail dot com
    17 years ago
    This function sometimes just fails to work for no appearent reason, returning FALSE and not creating a symlink. Doing the same with exec('ln -s source dest') works perfectly. I recommend using the latter.
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    -2
    Porjo
    15 years ago
    Keep in mind when using a shared filesystem such as NFS, that you probably don't want to create a symbolic link with absolute paths e.g.

    On Server1 you are running a PHP script that needs to create a symbolic link called widget2 which links to widget1 inside an NFS share mounted on your localhost at /mnt/nfs/widgets

    On Server2 this same NFS share is mounted under /usr/local/widgets

    If you used absolute paths on Server1, then Server2 would try to reference /mnt/nfs/widgets/widget1 which it won't be able to find...

    You need to cd into the directory first, then create the link - that way the link will be relative. Unless you run the PHP script from the same directory where you'll be creating the symbolic links, then you can't use symlink(). Use exec() instead as follows:

    <?php

    exec    ("cd <nfs mount path>; ln -s <target> <link_name>");

    ?>
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    -2
    contact at tcknetwork dot com
    19 years ago
    To make your code portable on unix AND win32, do the following
    1. Download http://www.dynawell.com/reskit/microsoft/win2000/linkd.zip
    2. Unzip, put linkd.exe in C:\Windows\System32 or C:\WINNT\System32
    3. Include in your code the following
    <?
    function _syslink($t /*target*/ ,$l /*link*/ ) {
    if ($_SERVER["WINDIR"]) {
    $p=dirname($_SERVER["SCRIPT_FILENAME"])."/";
    exec("linkd ".$p.$t." ".$p.$l);
    } else syslink($t,$l);
    }
    function _unlink($l /*link*/ ) {
    if ($_SERVER["WINDIR"]) {
    $p=dirname($_SERVER["SCRIPT_FILENAME"])."/";
    exec("linkd ".$p.$l." /D");
    } else unlink($l);
    }
    ?>
    4. Enjoy
    _symlink(TARGET,LINK) works like symlink() on *nix
    _unlink(LINK) to delete properly the link created
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    -2
    Anonymous
    18 years ago
    Olszewski seems pretty good, but just to boost the security a bit, the fifth line of his script should read
    <?
    ...
    $q = rand(1,26);
    ...
    ?>

    instead of rand(1,24).
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