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NULL

Специальное значение NULL говорит о том, что эта переменная не имеет значения. NULL - это единственно возможное значение типа NULL.

Замечание: Пустой тип был введен в PHP 4

Переменная считается NULL если

  • ей была присвоена константа NULL.

  • ей еще не было присвоено какое-либо значение.

  • она была удалена с помощью unset().

Синтаксис

Существует только одно значение типа NULL - регистро-независимое ключевое слово NULL.

<?php
$var 
NULL;       
?>

Смотрите также is_null() и unset().


User Contributed Notes
NULL
james
20-Sep-2007 03:25
A little speed test:

<?php
$v
= NULL;

$s = microtime(TRUE);
for(
$i=0; $i<1000; $i++) {
   
is_null($v);
}
print
microtime(TRUE)-$s;
print
"<br>";

$s = microtime(TRUE);
for(
$i=0; $i<1000; $i++) {
   
$v===NULL;
}
print
microtime(TRUE)-$s;
?>

Results:

0.017982006072998
0.0005950927734375

Using "===" is 30x quicker than is_null().
nl-x at bita dot nl
09-Jul-2007 05:33
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.
cdcchen at hotmail dot com
26-May-2006 03:17
empty() is_null() !isset()

$var = "";

empty($var) is true.
is_null($var) is false.
!isset($var) is false.
06-Jan-2006 09:51
// Difference between "unset($a);" and "$a = NULL;" :
<?php
// unset($a)
$a = 5;
$b = & $a;
unset(
$a);
print
"b $b "; // b 5

// $a = NULL; (better I think)
$a = 5;
$b = & $a;
$a = NULL;
print
"b $b "; // b
print(! isset($b)); // 1
?>
poutri_j at epitech dot net
26-Jul-2005 11:56
if you declare something like this :

<?php
class toto
{
    public
$a = array();

    public function
load()
    {
        if (
$this->a == null) // ==> the result is true
           
$a = other_func();
    }

}
?>

be carefull, that's strange but an empty array is considered as a null variable
rizwan_nawaz786 at hotmail dot com
19-Oct-2004 04:22
Hi
 Rizwan Here
  
   Null is the Constant in PHP. it is use to assign a empty value to the variable like

  $a=NULL;

  At this time $a has is NULL or $a has no value;

  When we declaire a veriable in other languages than that veriable has some value depending on the value of memory location at which it is pointed but in php when we declaire a veriable than php assign a NULL to a veriable.
dward at maidencreek dot com
12-Nov-2001 11:52
Nulls are almost the same as unset variables and it is hard to tell the difference without creating errors from the interpreter:

<?php
$var
= NULL;
?>

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.
tbdavis at greyshirt dot net
11-Oct-2001 11:36
Unlike the relational model, NULL in PHP has the following properties:
NULL == NULL is true,
NULL == FALSE is true.
And in line with the relational model, NULL == TRUE fails.
 

 
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