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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

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 8 notes

5 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
nl-x at bita dot nl
9 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.
kuzawinski dot marcin at NOSPAM dot gmail dot com
2 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".
sebastian dot goendoer at NOSPAM dot telekom dot de
1 year ago
Valiable values of (almost) all types that can be juggled to something "like" NULL will eval to NULL:

['_string'] = "";
$test['_int'] = 0;
$test['_float'] = 0.0;
$test['_null'] = null;
$test['_arr'] = array();
$test['_obj'] = new \stdClass();

$test as $key => $value)
$key.": ";
$value == null)
"is null\n";
"is not null\n";

will give you

_string: is null
_int: is null
_float: is null
_null: is null
_arr: is null
_obj: is not null
3 years ago
Be careful using NULL together with namespaces. If a NULL constant is redefined in a namespace other than global, you will get unexpected results when comparing to NULL inside the namespace. Instead always use \NULL, \FALSE, and \TRUE when comparing. Otherwise it may lead to application failures and potential security issues where certain checks could be effectively disabled.

A simple example to demonstrate the behavior:

namespace RedefinedConstants {

// redefining global namespace constants has no effect
define('NULL', 'I am not global NULL!');
define('TRUE', 'I am not global TRUE!');
define('FALSE', 'I am not global FALSE!');

// redefining local namespace constants will work
define('RedefinedConstants\NULL', 'I am not NULL!', \TRUE);
define('RedefinedConstants\FALSE', 'I am not FALSE!', \TRUE);
define('RedefinedConstants\TRUE', 'I am not TRUE!', \TRUE);

NULL, \NULL, null, \null, Null, \Null,
FALSE, \FALSE, false, \false, False, \False,
TRUE, \TRUE, true, \true, True, \True

foxdie_cs at hotmail dot com
4 years ago
a quick note about the magic function __get() :

class Foo{
    public function
$this->bar = NULL;
var_dump( $this->bar ); //prit 'NULL' but won't call the magic method __get()
unset( $this->bar );
var_dump( $this->bar ); //print 'GET bar' and 'NULL'
    public function
__get( $var ){ echo "GET " . $var; }

dward at maidencreek dot com
15 years ago
Nulls are almost the same as unset variables and it is hard to tell the difference without creating errors from the interpreter:


isset($var) is FALSE
empty($var) is TRUE
is_null($var) is TRUE

isset($novar) is FALSE
empty($novar) is TRUE
is_null($novar) gives an Undefined variable error

$var IS in the symbol table (from get_defined_vars())
$var CAN be used as an argument or an expression.

So, in most cases I found that we needed to use !isset($var) intead of is_null($var) and then set $var = NULL if the variable needs to be used later to guarantee that $var is a valid variable with a NULL value instead of being undefined.
11 years ago
// Difference between "unset($a);" and "$a = NULL;" :
// unset($a)
$a = 5;
$b = & $a;
"b $b "; // b 5

// $a = NULL; (better I think)
$a = 5;
$b = & $a;
$a = NULL;
"b $b "; // b
print(! isset($b)); // 1
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