unset

Description

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);

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;
      

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.

function foo(&$bar) { 
    unset($bar); 
    $bar = "blah"; 
}

$bar = 'something';
echo "$bar\n";

foo($bar);
echo "$bar\n";
      

something
something
      

If a static variable is unset() inside of a function, unset() unsets the reference to the static variable, rather than the static variable itself.

function foo() { 
    static $a;
    $a++;
       echo "$a\n";

    unset($a);
}

foo();
foo();
foo();
      

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:

function foo() {
    unset($GLOBALS['bar']);       
}

$bar = "something";
foo();
      

See also isset() and empty().