Useless Unset

Unset is the function to use to destroy a variable. unset is also an operator (operator), which actually does the same as the function.

Unsetting variable have different effect, depending on the type of variable. The five following cases make it useless to unset a variable.

• Unsetting a passed-by-value argument : arguments that are passed by value are copied from the calling context to the current one. Destroying the local variable is pointless : it will be destroyed (whatever that means), at the end of the method.
• Unsetting a passed-by-reference argument : this time again, the actual value is left untouched. The reference itself is destroyed, but the value is still available in the original context.
• Unsetting a global variable : inside a function, variable that are global can't be destroyed in the global scope by unsetting them. The only solution is to unset them in the $GLOBALS variable.
• Unsetting a static variable : in a function, a static variable is a variable will survive the end of the current call, and be available in the next call. Unsetting the variable will simply destroy it in the current call : it will be recreated at the next call.
• Unsetting a blind variable in a foreach loop : PHP works on a copy of the original array, and loops over the elements in that copy. Destroying such a variable, index or value, is useless. On the other hand, using the index to destroy the value in the original array is fine.

Here is a pot-pourri of the previous situations :

<?php

function foo(&$argByReference, $argByValue) { 
	static $theStatic;
	global $theGlobal;
	
	unset($theStatic); // useless, will be back next call
	unset($theGLobal); // useless, should unset $GLOBALS['theGlobal'];
	unset($argByReference); // useless, will only destroy local reference
	unset($argByValue); // useless, will only destroy local copy 
	
	foreach($array as $key => $value) {
		unset($value);
	}
	
}

$varByReference = 1;
$varByValue = 2;
foo($varByReference, $varByValue);

?>

With a positive view, unset may be used to unset local variables in a function, global variables in the global scope, object properties and array index.

It is recommended to avoid using unset on resilient variables.

Rule Details

The following code is considered a warning:

<?php

function foo(&$argByReference, $argByValue) { 
	static $theStatic;
	global $theGlobal;
	
	unset($theStatic); // useless, will be back next call
	unset($theGLobal); // useless, should unset $GLOBALS['theGlobal'];
	unset($argByReference); // useless, will only destroy local reference
	unset($argByValue); // useless, will only destroy local copy 
	
	foreach($array as $key => $value) {
		unset($value);
	}

}

$varByReference = 1;
$varByValue = 2;
foo($varByReference, $varByValue);

?>

The following pattern is considered legit:

<?php

unset ($variableInGlobalScope);
unset ($GLOBALS['anyGlobalVariable']);

function foo() {
	$localVariable = array(1);
	
	unset($localVariable[0]);
	unset($localVariable);
	
	unset($this->property);
	
	foreach($array as $key => $value) {
		unset($array[$key]);
	}
}

?>

Further Reading

• unset