PHP 8.4.0 RC4 available for testing

NULO

El valor especial null representa una variable sin valor. null es el único valor posible del tipo null.

Una variable es considerada null si:

  • se le ha asignado la constante null.

  • no se le ha asignado un valor todavía.

  • se ha destruido con unset().

Sintaxis

No hay más que un valor de tipo null, y es la constante null insensible a mayúsculas/minúsculas.

<?php
$var
= NULL;
?>

Véase también las funciones is_null() y unset().

La conversión a null

Advertencia

Esta característica ha sido declarada OBSOLETA a partir de PHP 7.2.0. Su uso está totalmente desaconsejado.

Convertir una variable a null usando (unset) $var no eliminará la variable ni destruirá su valor. Sólo retornará un valor null.

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User Contributed Notes 2 notes

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71
quickpick
13 years ago
Note: empty array is converted to null by non-strict equal '==' comparison. Use is_null() or '===' if there is possible of getting empty array.

$a = array();

$a == null <== return true
$a === null < == return false
is_null($a) <== return false
up
47
Hayley Watson
6 years ago
NULL is supposed to indicate the absence of a value, rather than being thought of as a value itself. It's the empty slot, it's the missing information, it's the unanswered question. It's not a jumped-up zero or empty set.

This is why a variable containing a NULL is considered to be unset: it doesn't have a value. Setting a variable to NULL is telling it to forget its value without providing a replacement value to remember instead. The variable remains so that you can give it a proper value to remember later; this is especially important when the variable is an array element or object property.

It's a bit of semantic awkwardness to speak of a "null value", but if a variable can exist without having a value, the language and implementation have to have something to represent that situation. Because someone will ask. If only to see if the slot has been filled.
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