void unset (mixed var [, mixed var [, ...]])
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
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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 $foo;
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bar
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If a variable that is PASSED BY REFERENCE is unset() inside of a function, only the local variable is destroyed. The variable in the calling environment will retain the same value as before unset() was called.
function foo(&$bar) {
unset($bar);
$bar = "blah";
}
$bar = 'something';
echo "$bar\n";
foo($bar);
echo "$bar\n";
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something
something
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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();
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1
2
3
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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();
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Note: unset() is a language construct.