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unset
(PHP 3, PHP 4)
Description
蛁: unset() is a language construct.
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.
瞰赽 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() destroyes the variable and all its references.
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().
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