PHP unset function

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unset

(PHP 3, PHP 4, PHP 5)

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

// 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);

Note:
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.

function destroy_foo() 
{
    global $foo;
    unset($foo);
}
 
$foo = 'bar';
destroy_foo();
echo $foo;

The above example will output:

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

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.

function foo(&$bar) 
{
    unset($bar);
    $bar = "blah";
}
 
$bar = 'something';
echo "$bar\n";
 
foo($bar);
echo "$bar\n";

The above example will output:

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.

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:

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().