The 7th Annual China PHP Conference


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 5 notes

8 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
1 year ago
Note: Non Strict Comparison '==' returns bool(true) for

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false
Hayley Watson
1 year 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.
kuzawinski dot marcin at NOSPAM dot gmail dot com
5 years ago
Funny. It looks like, that there is one, and only one possible value for variable $a that will pass this test:

($a != NULL) && ((bool)$a == NULL)

It's "0" and it works because casting string "0" to boolean gives FALSE (and it's the only non empty string, that works this way). So remember that casting is not "transitive".
nl-x at bita dot nl
11 years ago
Watch out. You can define a new constant with the name NULL with define("NULL","FOO");. But you must use the function constant("NULL"); to get it's value. NULL without the function call to the constant() function will still retrieve the special type NULL value.
Within a class there is no problem, as const NULL="Foo"; will be accessible as myClass::NULL.
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