unset
(PHP 4, PHP 5, PHP 7)
unset — Unset a given variable
Description
unset() destroys the specified variables.
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 will output:
bar
To unset() a global variable inside of a function, then 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";
?>
The above example will output:
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: $bar\n";
}
foo();
foo();
foo();
?>
The above example will output:
Before unset: 1, after unset: 23 Before unset: 2, after unset: 23 Before unset: 3, after unset: 23
Examples
Example #1 unset() example
<?php
// destroy a single variable
unset($foo);
// destroy a single element of an array
unset($bar['quux']);
// destroy more than one variable
unset($foo1, $foo2, $foo3);
?>
Example #2 Using (unset) casting
(unset) casting is often confused with the unset() function. (unset) casting serves only as a NULL-type cast, for completeness. It does not alter the variable it's casting. The (unset) cast is deprecated as of PHP 7.2.0.
<?php
$name = 'Felipe';
var_dump((unset) $name);
var_dump($name);
?>
The above example will output:
NULL string(6) "Felipe"
Notes
Note: Because this is a language construct and not a function, it cannot be called using variable functions.
Note:
It is possible to unset even object properties visible in current context.
Note:
It is not possible to unset $this inside an object method since PHP 5.
Note:
When using unset() on inaccessible object properties, the __unset() overloading method will be called, if declared.
See Also
- isset() - Determine if a variable is set and is not NULL
- empty() - Determine whether a variable is empty
- __unset()
- array_splice() - Remove a portion of the array and replace it with something else
- (unset) casting
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