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Псевдо-типы, используемые в этой документации Ресурс

Last updated: Sat, 27 Jan 2007

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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().

add a note User Contributed Notes
NULL
cdcchen at hotmail dot com
25-May-2006 08:17
`empty() is_null() !isset()
$var = "";
empty($var) is true.
is_null($var) is false.
!isset($var) is false.`
27-Apr-2006 03:16
`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 at gadz dot org
07-Mar-2006 10:37
`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 = '';", $var is set AND is equal to '', the empty string.`
zola at zolaweb dot com
14-Jan-2006 09:53
`Discovered something probably worth mentioning.
I had a form that had several values that didn't have to be set. I wanted to use the word "NULL" as a word (as opposed to the NULL constant) to go into the SQL statement when creating a new record.
If I do a check based on the variable having no value:
if ($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'] ." ', ' ".$array['some_other_var'] ." ')";
again, it treats NULL as the constant NULL instead of the word.
This seems inconsistent--one would have thought that enclosing it in double quotes would say I want the letters NULL as opposed to the constant, and I'll bug report it as well but wanted to mention it here for other users.`
06-Jan-2006 01:51
`// Difference between "unset($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
?>`
getphp at gmx dot net
04-Aug-2005 04:54
`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 at epitech dot net
26-Jul-2005 04:56
`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`
disappear at dissolution dot ath dot cx
02-May-2005 03:20
`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 ) <br><br>" ;
for ( $i = 0 ; $i < $ArrayCount ; $i++ )
{
if ( $Array [ $i ] == NULL )
{
$String .= '$Array [ $i ] == NULL :: $Array [ ' . $i . ' ] <br>' ;
}
if ( $Array [ $i ] === NULL )
{
$String .= '$Array [ $i ] === NULL :: $Array [ ' . $i . ' ] <br>' ;
}
if ( is_null ( $Array [ $i ] ) )
{
$String .= 'is_null ( $Array [ $i ] ) :: $Array [ ' . $i . ' ] <br>' ;
}
}
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 ]`
jaumesb aat consert doot net
26-Feb-2005 12:38
`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 )`
rizwan_nawaz786 at hotmail dot com
18-Oct-2004 09:22
`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 $a has no value;
When we declaire a veriable in other languages than that veriable has some value depending on the value of memory location at which it is pointed but in php when we declaire a veriable than php assign a NULL to a veriable.`
pozmu at wp dot pl
20-Apr-2004 08:21
`responding to joemamacow at hotmail dot com comment:
if you are using isset() to check is the variable set, the more logically and clear way to "delete" the variable is to use unset() (http://www.php.net/unset ) function:
<?php
unset($variable);
?>
Of course setiing variable value to NULL is also OK.`
alex at netflex dot nl
13-Jun-2002 04:13
`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;
}
}
?>`
avbentem at hetnet.nl
22-Feb-2002 01:25
`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(" . $desc . ");" ));
}
Note that even TRUE AND TRUE does not evaluate to a boolean value, and that OR and XOR behave different as well!
PHP Version: 4.0.6
false --> bool(false)
true --> bool(true)
null --> NULL
!null --> bool(true)
true and true --> int(1)
true and false --> int(0)
true or true --> int(1)
true or false --> int(1)
true xor true --> bool(false)
true xor false --> bool(true)
true == null --> bool(false)
true === null --> bool(false)
true != null --> bool(true)
true !== null --> bool(true)
false == null --> bool(true)
false === null --> bool(false)
false != null --> bool(false)
false !== null --> bool(true)
null == null --> bool(true)
null != null --> bool(false)
null === null --> bool(true)
null !== null --> bool(false)
null or null --> int(0)
null xor null --> bool(false)
null and null --> int(0)
true or null --> int(1)
true xor null --> bool(true)
true and null --> int(0)
false or null --> int(0)
false xor null --> bool(false)
false and null --> int(0)
true < null --> bool(false)
true > null --> bool(true)
false < null --> bool(false)
false > null --> bool(false)
1 + null --> int(1)
"text" . null --> string(4) "text"
Finally, for those who do not know SQL: in SQL the NULL value is evaluated a bit like "I do not know; it could be anything, like 0, 1, a, b, true, false or even nothing at all". This implies that in SQL NULL == NULL could be interpreted as "could be anything" == "could be something else", which does not yield true! Instead, it yields NULL, which boils down to FALSE in boolean context...
Likewise, in SQL:
NULL AND TRUE yields NULL
NULL OR TRUE yields TRUE
NULL AND FALSE yields FALSE
NULL OR FALSE yields NULL
NULL == TRUE yields FALSE
NULL == FALSE yields FALSE
a.`
sc at dbtech dot de
16-Feb-2002 03:59
`NULL as the best way to detect additional parameters of unknown type:
function FooBar($Param = NULL) {
if (is_null($Param)) {
[...]`
junk dot phpnet at gasolinemm dot com
17-Nov-2001 09:19
`$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 */`
dward at maidencreek dot com
12-Nov-2001 03:52
`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.`
tbdavis at greyshirt dot net
11-Oct-2001 04:36
`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.`
add a note
Псевдо-типы, используемые в этой документации Ресурс

Last updated: Sat, 27 Jan 2007
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