PHP unset() Function

The PHP unset() function is not really a function, but a language construct and used to unset the specified variables. The behavior of unset() inside of a function depends upon the type of variable which is attempted to be destroyed.

• 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.
• To unset() a global variable inside of a function, $GLOBALS array can be used.
• 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.
• 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.

Syntax

unset(var, vars)

Parameters

Return Value

No value is returned.

Example: unset() example

The example below shows the usage of unset() function.

<?php
//destroy a single variable
unset($test);

//destroy multiple variables
unset($test1, $test2, $test3);

//destroy a single element of an array
unset($arr['xyz']);
?>

Example: using unset() with global variable inside of a function

When a global 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. Consider the example below:

<?php
$var = 'Hello';

function destroy_var() {
  global $var;
  unset($var);
}

destroy_var();
echo "Value of \$var: ".$var;
?>

The output of the above code will be:

Value of $var: Hello

Example: unset() a global variable inside of a function

To unset() a global variable inside of a function, $GLOBALS array can be used. See the example below:

<?php
$var = 'Hello';

function destroy_var() {
  global $var;
  unset($GLOBALS['var']);
}

destroy_var();
echo "Value of \$var: ".$var;
?>

The output of the above code will be:

Value of $var: 
PHP Warning:  Undefined variable $var in Main.php on line 10

Example: unset() a passed by reference variable

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. Consider the example below:

<?php
function foo(&$var) {
  unset($var);
  $var = "World";
}

$var = 'Hello';
echo "$var\n";

foo($var);
echo "$var\n";
?>

The output of the above code will be:

Hello
Hello

Example: unset() a static variable inside of a function

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. See the example below:

<?php
function foo() {
  static $var;
  $var++;
  echo "Before unset: $var, ";
  unset($var);
  $var = 50;
  echo "After unset: $var\n";
}

foo();
foo();
foo();
?>

The output of the above code will be:

Before unset: 1, After unset: 50
Before unset: 2, After unset: 50
Before unset: 3, After unset: 50