リソース
This simple shorthand seems to work for setting new variables to NULL:
<?php
$Var;
?>
The above code will set $Var to NULL
UPDATE: After further testing it appears the code only works in the global scope and does not work inside functions.
<?php
function Example(){
$Var;
var_dump($Var);
}
?>
Would not work as expected.
NULL
特別な NULL 値は、ある変数が値を持たないことを表します。NULL は、NULL 型の唯一の値です。
変数は、以下の場合に NULL とみなされます。
-
定数 NULL が代入されている場合。
-
まだ値が何も代入されていない場合。
-
unset() されている場合。
NULL へのキャスト
(unset) $var を使って変数を null にキャストすると、 その変数を削除したり値の設定を解除したりはしません。 単に NULL 値を返すだけです。
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User Contributed Notes
NULL
ryan at trezshard dot com
01-Jun-2011 08:31
quickpick
22-Apr-2011 03:36
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
james
20-Sep-2007 08:25
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 10:33
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
25-May-2006 08:17
empty() is_null() !isset()
$var = "";
empty($var) is true.
is_null($var) is false.
!isset($var) is false.
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
?>
poutri_j at epitech dot net
26-Jul-2005 04:56
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
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.
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:
<?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 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.