El tipo null es el tipo unidad de PHP, es decir, que solo tiene un valor:
null .
Las variables no definidas y unset() tendrán el
valor null .
Solo hay un valor de tipo null , y es la constante
insensible a mayúsculas y minúsculas null .
<?php
$var = NULL;
?>null Esta funcionalidad está OBSOLETA a partir de PHP 7.2.0, y ELIMINADA a partir de PHP 8.0.0. Depender de esta funcionalidad está altamente desaconsejado.
Convertir una variable a null utilizando la sintaxis (unset) $var
no borrará la variable, ni sobrescribirá su valor.
Solo devolverá el valor null .
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 falseNULL 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.