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PHP : Language Reference : Types : NULL

NULL

The special NULL value represents that a variable has no value. NULL is the only possible value of type NULL.

Note:

The null type was introduced in PHP 4.

A variable is considered to be NULL if

  • it has been assigned the constant NULL.

  • it has not been set to any value yet.

  • it has been unset().

Syntax

There is only one value of type NULL, and that is the case-insensitive keyword NULL.

<?php
$var
= NULL;
?>

See also is_null() and unset().

Related Examples ( Source code ) » language.types.null











Code Examples / Notes » language.types.null

nl-x

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.


tbdavis

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.


27-apr-2006 11:16

To zola at zolaweb dot com, if you are trying to default fields to NULL when empty you should just pass the empty string in the SQL statement and have the column in the table set up with a default value of NULL. Makes things a little less confusing. :-)

owk dot ch199_ph

To zola at zolaweb dot com :
"It treats NULL as the constant and unsets the variable."
It doesn't. As you are trying to find if your variable is empty, your code should be :
if (empty ($array['var'])) {
   $array['var'] = "NULL";
}
You know, when "!isset ($var)" is true, $var is already not set...
PS : when "$var = '';", $var is set AND is equal to '', the empty string.


avbentem

To extend a bit on tbdavis's comment:
:: NULL == NULL is true
:: NULL == FALSE is true
:: NULL == TRUE is false
However: note the implicit type conversions that PHP performs! When using 'identical' instead of 'equal' then both NULL === FALSE and NULL === TRUE yield FALSE. An overview is easily created using something like
   function evalExpr( $desc )
   {
     echo str_pad($desc , 15) . "--> ";
     var_dump( eval( "return(" . $desc . ");" ));
   }
Note that even TRUE AND TRUE does not evaluate to a boolean value, and that OR and XOR behave different as well!
   PHP Version: 4.0.6
   false          --> bool(false)
   true           --> bool(true)
   null           --> NULL
   !null          --> bool(true)
   true and true  --> int(1)
   true and false --> int(0)
   true or true   --> int(1)
   true or false  --> int(1)
   true xor true  --> bool(false)
   true xor false --> bool(true)

   true == null   --> bool(false)
   true === null  --> bool(false)
   true != null   --> bool(true)
   true !== null  --> bool(true)
   false == null  --> bool(true)
   false === null --> bool(false)
   false != null  --> bool(false)
   false !== null --> bool(true)
   null == null   --> bool(true)
   null != null   --> bool(false)
   null === null  --> bool(true)
   null !== null  --> bool(false)
   null or null   --> int(0)
   null xor null  --> bool(false)
   null and null  --> int(0)
 
   true or null   --> int(1)
   true xor null  --> bool(true)
   true and null  --> int(0)
   false or null  --> int(0)
   false xor null --> bool(false)
   false and null --> int(0)
   true < null    --> bool(false)
   true > null    --> bool(true)
   false < null   --> bool(false)
   false > null   --> bool(false)
   1 + null       --> int(1)
   "text" . null  --> string(4) "text"
Finally, for those who do not know SQL: in SQL the NULL value is evaluated a bit like "I do not know; it could be anything, like 0, 1, a, b, true, false or even nothing at all". This implies that in SQL NULL == NULL could be interpreted as "could be anything" == "could be something else", which does not yield true! Instead, it yields NULL, which boils down to FALSE in boolean context...
Likewise, in SQL:
   NULL AND TRUE yields NULL
   NULL OR TRUE yields TRUE
   NULL AND FALSE yields FALSE
   NULL OR FALSE yields NULL
   NULL == TRUE yields FALSE
   NULL == FALSE yields FALSE
a.


pozmu

responding to joemamacow at hotmail dot com comment:
if you are using isset() to check is the variable set, the more logically and clear way to "delete" the variable is to use unset() (http://www.php.net/unset ) function:
<?php
unset($variable);
?>
Of course setiing variable value to NULL is also OK.


dward

Nulls are almost the same as unset variables and it is hard to tell the difference without creating errors from the interpreter:
$var = NULL;
isset($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.


sc

NULL as the best way to detect additional parameters of unknown type:
function FooBar($Param = NULL) {
 if (is_null($Param)) {
 [...]


jaumesb aat consert doot net

Note that the expression
( $v == NULL )
will evaluate as TRUE if $v is zero or the empty string.
To avoid this, remember to use :
( $v === NULL )


getphp

in addition to poutri_j at epitech dot net:
you missed that ($var == null) dont respect the type ... try this:
<?php
$t = array();
if ($t === NULL OR is_null($t)) {
 echo "Value is NULL";
} elseif ($t === TRUE) {
 echo "Value is TRUE";
} elseif ($t == NULL OR empty($t)) {
 echo "Value not set or empty.";
}
?>
Output:
Value maybe not set or empty.
PS: Yes, some conditions are redundant. This was intended.


poutri_j

if you declare something like this :
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

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.


disappear

Hi,
Im using PHP 5.0.3
i wrote a small null study to test the cases here and this is the results i got
Code ::
<?php
   $Array = array ( 0 , '' , FALSE , NULL ) ;
   $ArrayCount = count ( $Array ) ;
   $String .= '$Array = ' . "array ( 0 , '' , FALSE , NULL )
" ;
   for ( $i = 0 ; $i < $ArrayCount ; $i++ )
   {
       if ( $Array [ $i ] == NULL )
       {
           $String .= '$Array [ $i ] == NULL :: $Array [ ' . $i . ' ]
' ;
       }
       if ( $Array [ $i ] === NULL )
       {
           $String .= '$Array [ $i ] === NULL :: $Array [ ' . $i . ' ]
' ;
       }
       if ( is_null ( $Array [ $i ] ) )
       {
           $String .= 'is_null ( $Array [ $i ] ) :: $Array [ ' . $i . ' ]
' ;
       }
   }
   echo $String ;
?>
Results ::
$Array = array ( 0 , '' , FALSE , NULL )
$Array [ $i ] == NULL :: $Array [ 0 ]
$Array [ $i ] == NULL :: $Array [ 1 ]
$Array [ $i ] == NULL :: $Array [ 2 ]
$Array [ $i ] == NULL :: $Array [ 3 ]
$Array [ $i ] === NULL :: $Array [ 3 ]
is_null ( $Array [ $i ] ) :: $Array [ 3 ]


alex

Hi,
Function for looking if it is a NULL
<?php
 $var = NULL;
 if (isnull("var")) {
   echo "var===NULL\n";
 } else {
   echo "var!==NULL\n";
 }
 if (isnull("test")) { // give FALSE, test is not set
   echo "test===NULL\n";
 } else {
   echo "test!==NULL\n";
 }
 $array['var'] = NULL;
 if (isnull("var", $array)) {
   echo "array['var']===NULL\n";
 } else {
   echo "array['var']!==NULL\n";
 }
 function isnull($var, $base = FALSE) {
   if ($base===FALSE) {
     $base = &$GLOBALS;
   } elseif (!is_array($base)) {
     return FALSE;
   }
   if ((array_key_exists($var, $base))&&($base[$var]===NULL)) {
     return TRUE;
   } else {
     return FALSE;
   }
 }
?>


cdcchen

empty() is_null() !isset()
$var = "";
empty($var) is true.
is_null($var) is false.
!isset($var) is false.


zola

Discovered something probably worth mentioning.
I had a form that had several values that didn't have to be set. I wanted to use the word "NULL" as a word (as opposed to the NULL constant) to go into the SQL statement when creating a new record.
If I do a check based on the variable having no value:
if ($array['var'] == "")
{
$array['var'] = "NULL";
}
Then $array['var'] contains the word "NULL" the way I want it to, BUT I have to be careful with the sql statement (more on this in a moment)
On the other hand, if I check via !isset()
if (!isset($array['var']))
{
$array['var'] = "NULL";
}
It treats NULL as the constant and unsets the variable.
In the SQL, if I am inserting and put it in as is:
$sql = "INSERT INTO mytable VALUES(NULL, ".$array['var'] .", ' ".$array['some_other_var'] ." ')";
the word NULL replaces $array['var'] as it should, but if I enclose the variable in single quotes (because maybe that variable, if it's set, will contain a space)
$sql = "INSERT INTO mytable VALUES(NULL, ' ".$array['var'] ." ', ' ".$array['some_other_var'] ." ')";
again, it treats NULL as the constant NULL instead of the word.
This seems inconsistent--one would have thought that enclosing it in double quotes would say I want the letters NULL as opposed to the constant, and I'll bug report it as well but wanted to mention it here for other users.


james

A little speed test:
<?php
$v = NULL;
$s = microtime(TRUE);
for($i=0; $i<1000; $i++) {
is_null($v);
}
print microtime(TRUE)-$s;
print "
";
$s = microtime(TRUE);
for($i=0; $i<1000; $i++) {
$v===NULL;
}
print microtime(TRUE)-$s;
?>
Results:
0.017982006072998
0.0005950927734375
Using "===" is 30x quicker than is_null().


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
?>


junk dot phpnet

$a = "";
$b = NULL;
$a == $b;
/* returns true: $a has been converted to $b for the equality comparison */
is_null($a); //returns false
is_null($b); //returns true
$a === $b;
/* returns false: $a is not _identical_ to $b */


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