CascadiaPHP 2024


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().


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


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

La conversión a null


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

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
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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