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Last updated: Sat, 27 Jan 2007

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# unset

(PHP 3, PHP 4, PHP 5)

unset -- Unset a given variable

## Description

void unset ( mixed var [, mixed var [, mixed ...]] )

unset() destroys the specified variables. Note that in PHP 3, unset() will always return `TRUE` (actually, the integer value 1). In PHP 4, however, unset() is no longer a true function: it is now a statement. As such no value is returned, and attempting to take the value of unset() results in a parse error.

Пример 1. unset() example

`<?php
// destroy a single variable
unset($foo);
// destroy a single element of an array
unset($bar['quux']);
// destroy more than one variable
unset($foo1, $foo2, $foo3);
?>`

> Замечание: It is possible to unset even object properties visible in current context.

The behavior of unset() inside of a function can vary depending on what type of variable you are attempting to destroy.

If a globalized variable is unset() inside of a function, only the local variable is destroyed. The variable in the calling environment will retain the same value as before unset() was called.

`<?php
function destroy_foo()
{
global $foo;
unset($foo);
}
$foo = 'bar';
destroy_foo();
echo $foo;
?>`

Результат выполнения данного примера:

    bar

If you would like to unset() a global variable inside of a function, you can use the `$GLOBALS` array to do so:

`<?php
function foo()
{
unset($GLOBALS['bar']);
}
$bar = "something";
foo();
?>`

If a variable that is PASSED BY REFERENCE is unset() inside of a function, only the local variable is destroyed. The variable in the calling environment will retain the same value as before unset() was called.

`<?php
function foo(&$bar)
{
unset($bar);
$bar = "blah";
}
$bar = 'something';
echo "$bar\n";
foo($bar);
echo "$bar\n";
?>`

Результат выполнения данного примера:

    something
    something

If a static variable is unset() inside of a function, unset() destroys the variable only in the context of the rest of a function. Following calls will restore the previous value of a variable.

`<?php
function foo()
{
static $bar;
$bar++;
echo "Before unset: $bar, ";
unset($bar);
$bar = 23;
echo "after unset: $bar\n";
}
foo();
foo();
foo();
?>`

Результат выполнения данного примера:

    Before unset: 1, after unset: 23
    Before unset: 2, after unset: 23
    Before unset: 3, after unset: 23

> Замечание: Поскольку это языковая конструкция, а не функция, она не может вызываться при помощи переменных функций

See also isset(), empty(), and array_splice().

add a note User Contributed Notes
unset
09-Feb-2007 08:10
`Just to confirm, USING UNSET CAN DESTROY AN ENTIRE ARRAY. I couldn't find reference to this anywhere so I decided to write this.
The difference between using unset and using $myarray=array(); to unset is that obviously the array will just be overwritten and will still exist.
<?php
$myarray=array("Hello","World");
echo $myarray[0].$myarray[1];
unset($myarray);
//$myarray=array();
echo $myarray[0].$myarray[1];
echo $myarray;
?>
Output with unset is:
<?
HelloWorld
Notice: Undefined offset: 0 in C:\webpages\dainsider\myarray.php on line 10
Notice: Undefined offset: 1 in C:\webpages\dainsider\myarray.php on line 10
Output with $myarray=array(); is:
?>
<?
HelloWorld
Notice: Undefined offset: 0 in C:\webpages\dainsider\myarray.php on line 10
Notice: Undefined offset: 1 in C:\webpages\dainsider\myarray.php on line 10
Array
?>`
hessodreamy at gmail dot com
17-Jan-2007 03:51
`To clarify what hugo dot dworak at gmail dot com said about unsetting things that aren't already set:
unsetting a non-existent key within an array does NOT throw an error.
<?
$array = array();
unset($array[2]);
//this does not throw an error
unset($array[$undefinedVar]);
//Throws an error because of the undefined variable, not because of a non-existent key.
?>`
dibakar dot datta at gmail dot com
31-Mar-2006 11:31
`Instead of using the unset function for unregistering your session or other array values you can also do this samll feature and get this task done with just 1 line code.
Suppose, if you like to unregister your session store values.
You can use:
$_SESSION = array();
Well this syntax saves lot's of time instead of unsetting each values.`
hugo dot dworak at gmail dot com
26-Dec-2005 06:23
`If one tries to unset a typical variable that does not exist, no errors, warning or noticies will occur. However, if one tries to unset a non-existent array or an array with non-existent key, this will result in a notice. For instance:
<?php
$true = true;
$array = array ();
unset ($true, $undefinedVariable, $array [$undefinedKey], $undefinedArray [$undefinedKey]);
?>
The output is (PHP 5.0.5):
Notice: Undefined variable: undefinedKey
Notice: Undefined variable: undefinedKey
Notice: Undefined variable: undefinedArray`
clark at everettsconsulting dot com
11-Sep-2005 12:50
`In PHP 5.0.4, at least, one CAN unset array elements inside functions from arrays passed by reference to the function.
As implied by the manual, however, one can't unset the entire array by passing it by reference.
<?php
function remove_variable (&$variable) // pass variable by reference
{
unset($variable);
}
function remove_element (&$array, $key) // pass array by reference
{
unset($array[$key]);
}
$scalar = 'Hello, there';
echo 'Value of $scalar is: ';
print_r ($scalar); echo '<br />';
// Value of $scalar is: Hello, there
remove_variable($scalar); // try to unset the variable
echo 'Value of $scalar is: ';
print_r ($scalar); echo '<br />';
// Value of $scalar is: Hello, there
$array = array('one' => 1, 'two' => 2, 'three' => 3);
echo 'Value of $array is: ';
print_r ($array); echo '<br />';
// Value of $array is: Array ( [one] => 1 [two] => 2 [three] => 3 )
remove_variable($array); // try to unset the array
echo 'Value of $array is: ';
print_r ($array); echo '<br />';
// Value of $array is: Array ( [one] => 1 [two] => 2 [three] => 3 )
remove_element($array, 'two'); // successfully remove an element from the array
echo 'Value of $array is: ';
print_r ($array); echo '<br />';
// Value of $array is: Array ( [one] => 1 [three] => 3 )
?>`
no at spam dot com
30-Aug-2005 03:22
`In addition to what timo dot hummel at 4fb dot de said;
>For the curious: unset also frees memory of the variable used.
>
>It might be possible that the in-memory size of the PHP Interpreter isn't reduced, but your scripts won't touch the memory_limit boundary. Memory is reused if you declare new variables.
It might be worth adding that functions apparently don't free up memory on exit the same way unset does..
Maybe this is common knowledge, but although functions destroys variables on exit, it (apparently) doesn't help the memory.
So if you use huge variables inside functions, be sure to unset them if you can before returning from the function.
In my case, if I did not unset before return, then the script would use 20 MB more of memory than if I did unset.
This was tested with php 5.0.4 on apache 2 on windows xp, with no memory limit.
Before I did the test, I was under the impression that when you exit from functions, the memory used inside it would be cleared and reused. Maybe this should be made clear in the manual, for either unset() or in the chapter for functions.`
tom at diacope dot com
04-Aug-2005 03:51
`when working with $_SESSION or any other array like that and you want to delete part of the session array it's always worked for me to do:
$_SESSION['data'] = NULL;
unset($_SESSION['data']);`
muhamad_zakaria at yahoo dot com
04-Jul-2005 07:08
`We have experienced when we applied 'unset' to the overloaded properties (PHP5), consider the code below:
<?php
class TheObj {
public $RealVar1, $RealVar2, $RealVar3, $RealVar4;
public $Var = array();
function __set($var, $val) {
$this->Var[$var] = $val;
}
function __get($var) {
if(isset($this->Var[$var])) return $this->Var[$var];
else return -1;
}
}
$SomeObj = new TheObj;
// here we set for real variables
$SomeObj->RealVar1 = 'somevalue';
$SomeObj->{'RealVar2'} = 'othervalue';
$SomeObj->{'RealVar'.(3)} = 'othervaluetoo';
$SomeObj->{'RealVar'.'4'} = 'anothervalue';
// and here we set for virtual variables
$SomeObj->Virtual1 = 'somevalue';
$SomeObj->{'Virtual2'} = 'othervalue';
$SomeObj->{'Virtual'.(3)} = 'othervaluetoo';
$SomeObj->{'Virtual'.'4'} = 'anothervalue';
// now we will try to unset these variables
unset($SomeObj->RealVar1);
unset($SomeObj->{'RealVar'.(3)});
//the lines below will catch by '__get' magic method since these variables are unavailable anymore
print $SomeObj->RealVar1."\n";
print $SomeObj->{'RealVar'.(3)}."\n";
// now we will try to unset these variables
unset($SomeObj->Virtual1);
unset($SomeObj->{'Virtual'.(3)});
//but, these variables are still available??? eventhough they're "unset"-ed
print $SomeObj->Virtual1."\n";
print $SomeObj->{'Virtual'.(3)}."\n";
?>
Please note that PHP doesn't have magic callback to unset overloaded properties. This is the reason why unset($SomeObj->Virtual1) doesn't work.
But it does work when we set 'null' value such as the following code:
<?php
// now we will set 'null' value instead of using unset statement
$SomeObj->Virtual1 = null;
$SomeObj->{'Virtual'.(3)} = null;
// and now these variables are no longer available
print $SomeObj->Virtual1."\n";
print $SomeObj->{'Virtual'.(3)}."\n";
?>
Sound ugly, yeah?
This applied to the "virtual" array variable too, see more at http://bugs.php.net/bug.php?id=33513 (at feedback) about it.
PS: we used PHP version 5.1.0-dev from the CVS snapshot when we wrote the above codes.`
smyle at mailbox dot hu
25-May-2005 11:21
`This work for me only _SESSION in quotes:
unset($GLOBALS['_SESSION'][$sessionVariableName]);`
franckraynal at free dot fr
26-Feb-2005 05:02
`Here is another way to make 'unset' work with session variables from within a function :
<?php
function unsetSessionVariable ($sessionVariableName) {
unset($GLOBALS[_SESSION][$sessionVariableName]);
}
?>
May it work with others than me...
F.`
bedbin at gmail dot com
02-Feb-2005 06:33
`usefull tip:
if you have session variables like these.
<?php
echo "<pre>";
$_SESSION["num"] = array(1,2,3,4);
var_dump($_SESSION);
echo "-<br>";
unset($_SESSION);
var_dump($_SESSION);
?>
gives out:
array(1) {
["num"]=>
array(4) {
[0]=>
int(1)
[1]=>
int(2)
[2]=>
int(3)
[3]=>
int(4)
}
}
-
NULL
if you use empty instead unset you get same output as first var_dump($_SESSION) gives.
I hope help sb.`
Nghia
05-Jan-2005 08:41
`I saw this mentioned somewhere else but if you do
$var = NULL
then I've noticed less memory usuage than with unset(). In fact, unset didn't do anything.
This might be useful if you're doing a php-gtk app, thats starting to consume significant memory over a long period of time. This was the code I used to test
// Check memory before here
for($i = 0; $i < 100; $i++)
{
$dialog = &new GtkDialog();
$dialog->realize();
$dialog->destroy();
$dialog = NULL;
//unset($dialog);
}
// Check memory after here
Doing a difference between after and before results in:
Using destroy() and unset() -> ~31kb
Using $dialog = NULL -> ~13 kb
The expected memory usuage should be 0kb or around there.`
harycary at netscape dot net
14-Dec-2004 09:56
`If you ever have to unset all the variables of a class from within a funciton of that class use the following code:
<?php
class User
{
function User_login ( ... )
{...}
function User_logout ( $greeting )
{
...
foreach ( array_keys ( get_object_vars ( &$this ) ) as $val)
{ unset( $this->$val ); }
$this->greeting = $greeting;
...
}
}
?>
If anyone knows of a more effective way please post a reply.`
mv at brasil dot com
08-Nov-2004 08:04
`If you want to remove one element of Query String use this function, than place the returned values in <a href="script.php?'. remove_query("arg1") .'">
function remove_query($key) {
$arrquery = explode("&", $_SERVER["QUERY_STRING"]);
foreach ($arrquery as $query_value) {
$valor = substr($query_value, strpos($query_value, "=") + 1);
$chave = substr($query_value, 0, strpos($query_value, "="));
$querystring[$chave] = $valor;
}
unset($querystring[$key]);
foreach ($querystring as $query_key => $query_value) {
$query[] = "{$query_key}={$query_value}";
}
$query = implode("&", $query);
return $query;
}`
dan AT --nospam-- cubeland DOT co DOT uk
04-Nov-2004 11:38
`dh at argosign dot de -
it is possible to unset globals from within functions thanks to the $GLOBALS array:
<?php
$x = 10;
function test() {
// don't need to do ' global $x; '
unset ($GLOBALS['x']);
echo 'x: ' . $GLOBALS['x'] . '<br />';
}
test();
echo "x: $x<br />";
// will result in
/*
x:
x:
*/
?>`
timo dot hummel at 4fb dot de
07-Sep-2004 06:24
`For the curious: unset also frees memory of the variable used.
It might be possible that the in-memory size of the PHP Interpreter isn't reduced, but your scripts won't touch the memory_limit boundary. Memory is reused if you declare new variables.`
thorry at thorry dot net
05-Aug-2004 02:15
`The documentation is not entirely clear when it comes to static variables. It says:
If a static variable is unset() inside of a function, unset() destroys the variable and all its references.
<?php
function foo()
{
static $a;
$a++;
echo "$a\n";
unset($a);
}
foo();
foo();
foo();
?>
The above example would output:
1
2
3
And it does! But the variable is NOT deleted, that's why the value keeps on increasing, otherwise the output would be:
1
1
1
The references are destroyed within the function, this handeling is the same as with global variables, the difference is a static variable is a local variable.
Be carefull using unset and static values as the output may not be what you expect it to be. It appears to be impossible to destroy a static variable. You can only destroy the references within the current executing function, a successive static statement will restore the references.
The documentation would be better if it would say:
"If a static variable is unset() inside of a function, unset() destroys all references to the variable. "
Example: (tested PHP 4.3.7)
<?php
function foo()
{
static $a;
$a++;
echo "$a\n";
unset($a);
echo "$a\n";
static $a;
echo "$a\n";
}
foo();
foo();
foo();
?>
Would output:
1
1
2
2
3
3`
anon at no spam dot no address dot com
16-Jul-2004 09:19
`Adding on to what bond at noellebond dot com said, if you want to remove an index from the end of the array, if you use unset, the next index value will still be what it would have been.
Eg you have
<?php
$x = array(1, 2);
for ($i = 0; $i < 5; $i++)
{
unset($x[(count($x)-1)]); //remove last set key in the array
$x[] = $i;
}
?>
You would expect:
Array([0] => 1, [1] => 4)
as you want it to remove the last set key....
but you actually get
Array ( [0] => 1 [4] => 2 [5] => 3 [6] => 4 )
This is since even though the last key is removed, the auto indexing still keeps its previous value.
The only time where this would not seem right is when you remove a value off the end. I guess different people would want it different ways.
The way around this is to use array_pop() instead of unset() as array_pop() refreshes the autoindexing thing for the array.
<?php
$x = array(1, 2);
for ($i = 0; $i < 5; $i++)
{
array_pop($x); // removes the last item in the array
$x[] = $i;
}
?>
This returns the expected value of x = Array([0] => 1, [1] => 4);
Hope this helps someone who may need this for some odd reason, I did.`
bond at noellebond dot com
26-May-2004 02:34
`Note that though global arrays will not be altered by a function, an array in an object WILL be altered if referenced within one of its methods. For example:
function remove_index ($i)
{
unset($this->test_array[$i]);
$temp_array = array_values($this->test_array);
$this->test_array = $temp_array;
}
Will remove key $i from the object's array and reindex it.`
andre at twg dot com dot au
06-Mar-2004 09:16
`Only This works with register_globals being 'ON'.
unset( $_SESSION['variable'] );
The above will not work with register_globals turned on (will only work outside of a function).
$variable = $_SESSION['variable'];
unset( $_SESSION['variable'], $variable );
The above will work with register_globals on & inside a function`
warhog at warhog dot net
27-Jan-2004 08:52
`you may wan't to unset all variables which are defined, here's one way:
<?php
function unset_all_vars($a)
{ foreach($a as $key => $val)
{ unset($GLOBALS[$key]); }
return serialize($a); }
unset_all_vars(get_defined_vars());
?>
you can also save than a serialized var of the "memory" and perhaps store this in a temporary file.. very usefull if you work with text files and/or file uploads when you've got very large variables.
greetz`
kdechant at midwestarts dot com
23-Nov-2003 03:47
`As of PHP version 4.3.3, unset() results in a parse error if it is used with the @ error suppression operator.
For example:
@unset($var); // parse error
unset(@$var); // parse error
unset($var); // okay`
frank at agentbrand dot com
09-Nov-2003 09:59
`Use array_values() after unset() to reindex your array.
Note that unset() removes the index as a key, you will need to reindex your array again to get expected behavior`
vmizuba at queens dot org
27-Oct-2003 07:25
`for what it's worth...
in php 4.1, using unset to destroy a session variable, i.e. unset($_SESSION['variable']); destroys it by erasing variable information but leaves behind the variable name appended with a '!' in front of the name in the session file... leaving the session file larger and x bytes wasted depending on the variable name length`
add a note
var_dump unserialize

Last updated: Sat, 27 Jan 2007
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