PHP Reference

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.

Example 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); ?>

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;

?>

The above example would output:

bar

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";

?>

The above example would output:

something
something

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

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();

?>

Note: Because this is a language construct and not a function, it cannot be called using variable functions

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