What is set and unset in php?

(PHP 4, PHP 5, PHP 7, PHP 8)

unset — Unset a given variable

Description

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;
?>

The above example will output:

To unset() a global variable inside of a function, then use the $GLOBALS array to do so:

function foo() 
{
    unset($GLOBALS['bar']);
}$bar = "something";
foo();
?>

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";
?>

The above example will output:

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.

function foo()
{
    static $bar;
    $bar++;
    echo "Before unset: $bar, ";
    unset($bar);
    $bar = 23;
    echo "after unset: $bar\n";
}foo();
foo();
foo();
?>

The above example will output:

Before unset: 1, after unset: 23
Before unset: 2, after unset: 23
Before unset: 3, after unset: 23

Parameters

The variable to be unset.

Further variables.

Return Values

No value is returned.

Examples

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);
?>

Example #2 Using (unset) casting

(unset) casting is often confused with the unset() function. (unset) casting serves only as a NULL-type cast, for completeness. It does not alter the variable it's casting. The (unset) cast is deprecated as of PHP 7.2.0, removed as of 8.0.0.

$name = 'Felipe';var_dump((unset) $name);
var_dump($name);
?>

The above example will output:

Notes

Note: Because this is a language construct and not a function, it cannot be called using variable functions, or named arguments.

Note:

It is possible to unset even object properties visible in current context.

Note:

It is not possible to unset $this inside an object method.

Note:

When using unset() on inaccessible object properties, the __unset() overloading method will be called, if declared.

See Also

• isset() - Determine if a variable is declared and is different than null
• empty() - Determine whether a variable is empty
• __unset()
• array_splice() - Remove a portion of the array and replace it with something else
• (unset) casting

Hayley Watson ¶

9 years ago

You don't need to check that a variable is set before you unset it.
unset($a);
?>
is harmless.
if(isset($a)) {
    unset($a);
}
?>
is pointless complication.

This doesn't apply to properties of objects that have __isset() methods that visibly change object state or __unset() methods that don't properly check their arguments or have extra side effects.

The latter case means that __unset shouldn't do more than what it says on the tin, and also has the responsibility for checking (possibly using __isset()) that what it's being asked to do makes sense.

The former case is just plain bad design.

nox at oreigon dot de ¶

14 years ago

if you try to unset an object, please be careful about references.

Objects will only free their resources and trigger their __destruct method when *all* references are unsetted.
Even when they are *in* the object... sigh!

class A {
  function __destruct() {
    echo "cYa later!!\n";
  }
}$a = new A();
$a -> a = $a;
#unset($a); # Just uncomment, and you'll seeecho "No Message ... hm, what now?\n";
unset($a -> a);
unset($a);

echo

Kai Kunstmann ¶

14 years ago

Since unset() is a language construct, it cannot be passed anything other than a variable. It's sole purpose is to "unset" this variable, ie. to remove it from the current scope and destroy it's associated data. This is true especially for reference variables, where not the actual value is destroyed but the reference to that value. This is why you can't wrap 'unset()' in a user defined function: You would either unset a copy of the data if the parameter is passed by value, or you would just unset the reference variable within the functions scope if the parameter is passed by reference. There is no workaround for that, as you cannot pass 'scope' to a function in PHP. Such a function can only work for variables that exist in a common or global scope (compare 'unset($_GLOBALS[variable])').

I don't know how PHP handles garbage collection internally, but I guess this behavior can result in a huge memory leak: if a value variable goes out of scope with a second variable still holding a reference to the in-memory value, then unsetting that reference would still hold the value in memory but potentially unset the last reference to that in-memory data, hence: occupied memory that is rendered useless as you cannot reference it anymore.

j dot vd dot merwe at enovision dot net ¶

12 years ago

A sample how to unset array elements from an array result coming from a mysql request. In this sample it is checking if a file exists and removes the row from the array if it not exists.

$db->set_query("select * from documents where document_in_user = 0"); //1 
$documents = $db->result_to_array($db->get_result()); //1foreach ($documents as $key => $row) { //2$file     = "uploads/".rawurldecode($row['document_name']);

      if (

franckraynal at free dot fr ¶

17 years ago

Here is another way to make 'unset' work with session variables from within a function :

function unsetSessionVariable ($sessionVariableName) {
   unset($GLOBALS[_SESSION][$sessionVariableName]);
}
?>

May it work with others than me...
F.

andre at twg dot com dot au ¶

18 years ago

Only This works with register_globals being 'ON'.

unset( $_SESSION['variable'] );

The above will not work with register_globals turned on (will only work outside of a function).

$variable = $_SESSION['variable'];
unset( $_SESSION['variable'], $variable );

The above will work with register_globals on & inside a function

anon at no spam dot no address dot com ¶

18 years ago

Adding on to what bond at noellebond dot com said, if you want to remove an index from the end of the array, if you use unset, the next index value will still be what it would have been.

Eg you have
$x = array(1, 2);

for (

hessodreamy at gmail dot com ¶

15 years ago

To clarify what hugo dot dworak at gmail dot com said about unsetting things that aren't already set:

unsetting a non-existent key within an array does NOT throw an error.
$array = array();

unset($array[2]);
//this does not throw an error

unset($array[$undefinedVar]);
//Throws an error because of the undefined variable, not because of a non-existent key.
?>

pfreilly at umd dot umich dot edu ¶

10 years ago

Despite much searching, I have not yet found an explanation as to how one can manually free resources from variables, not so much objects, in PHP.  I have also seen many comments regarding the merits and demerits of unset() versus setting a variable to null.  Thus, here are the results of some benchmarks performed comparing unset() of numerous variables to setting them to null (with regards to memory usage and processing time):

10 variables:
Unset:
Memory Usage: 296
Time Elapsed: 1.0013580322266E-5

Null set:
Memory Usage: 1736
Time Elapsed: 5.9604644775391E-6

50 variables:
Unset:
Memory Usage: 296
Time Elapsed: 3.6001205444336E-5

Null set:
Memory Usage: 8328
Time Elapsed: 3.2901763916016E-5

100 variables:
Unset:
Memory Usage: 296
Time Elapsed: 5.6982040405273E-5

Null set:
Memory Usage: 15928
Time Elapsed: 5.8174133300781E-5

1000 variables:
Unset:
Memory Usage: 296
Time Elapsed: 0.00041294097900391

Null set:
Memory Usage: 168096
Time Elapsed: 0.00067591667175293

10000 variables:
Unset:
Memory Usage: 296
Time Elapsed: 0.0042569637298584

Null set:
Memory Usage: 1650848
Time Elapsed: 0.0076930522918701

100000 variables:
Unset:
Memory Usage: 296
Time Elapsed: 0.042603969573975

Null set:
Memory Usage: 16249080
Time Elapsed: 0.087724924087524

300000 variables:
Unset:
Memory Usage: 296
Time Elapsed: 0.13177299499512

Null set:
Memory Usage: 49796320
Time Elapsed: 0.28617882728577

Perhaps my test code for the null set was flawed, but despite that possibility it is simple to see that unset() has minimal processing time impact, and no apparent memory usage impact (unless the values returned by memory_get_usage() are flawed).  If you truly care about the ~4 microseconds saved over <50 variables, more power to you.  Otherwise, use unset() to minimize script impact on your system.
Note: Tested on PHP 5.3.8 installed via RPM on Fedora 14

clark at everettsconsulting dot com ¶

17 years ago

In PHP 5.0.4, at least, one CAN unset array elements inside functions from arrays passed by reference to the function.
As implied by the manual, however, one can't unset the entire array by passing it by reference.

function remove_variable (&$variable)  // pass variable by reference
{
    unset($variable);
}

function

phpmanual at kennel17 dot co dot uk ¶

12 years ago

Note that PHP 4 will generate a warning if you try to unset an array index that doesn't exist and whose parent doesn't exist.

Example:

  $foo

tecdoc at ukr dot net ¶

4 months ago

Be careful!, unset() element in array advances the internal array pointer one place forward

ray.paseur sometimes uses gmail ¶

6 months ago

Perhaps because it is a language construct and not a function, if you try to use the error suppression operator ...

@unset($var);

... you get this: Parse error: syntax error, unexpected 'unset' (T_UNSET) ...

Anonymous ¶

15 years ago

Just to confirm, USING UNSET CAN DESTROY AN ENTIRE ARRAY. I couldn't find reference to this anywhere so I decided to write this.

The difference between using unset and using $myarray=array(); to unset is that obviously the array will just be overwritten and will still exist.

$myarray

lion_cat at mail ru ¶

13 years ago

about unset for arrays

if you unset the last array member
$ar[0]==2
$ar[1]==7
$ar[2]==9

unset ($ar[2])

after addition a new member by $ar[]=7,

you will get
$ar[0]==2
$ar[1]==7
$ar[3]==7,

So, unset has no effect to internal array counter!!!

dan AT --nospam-- cubeland DOT co DOT uk ¶

17 years ago

dh at argosign dot de -
it is possible to unset globals from within functions thanks to the $GLOBALS array:

$x = 10;

function

thorry at thorry dot net ¶

18 years ago

The documentation is not entirely clear when it comes to static variables. It says:

If a static variable is unset() inside of a function, unset() destroys the variable and all its references.

function foo()
{
   static $a;
   $a++;
   echo "$a\n";
   unset($a);
}foo();
foo();
foo();
?>

The above example would output:

1
2
3

And it does! But the variable is NOT deleted, that's why the value keeps on increasing, otherwise the output would be:

1
1
1

The references are destroyed within the function, this handeling is the same as with global variables, the difference is a static variable is a local variable.

Be carefull using unset and static values as the output may not be what you expect it to be. It appears to be impossible to destroy a static variable. You can only destroy the references within the current executing function, a successive static statement will restore the references.

The documentation would be better if it would say:
"If a static variable is unset() inside of a function, unset() destroys all references to the variable. "

Example: (tested PHP 4.3.7)
function foo()
{
   static $a;
   $a++;
   echo "$a\n";
   unset($a);
   echo "$a\n";
   static $a;   
   echo "$a\n";
}foo();
foo();
foo();
?>

Would output:

1

1
2

2
3

3

pauljamescampbell at gmail dot com ¶

14 years ago

Here's my variation on the slightly dull unset method. It throws in a bit of 80's Stallone action spice into the mix. Enjoy!

/**
* function rambo (first blood)
*
* Completely and utterly destroys everything, returning the kill count of victims
*
* @param    It don't matter, it’s Rambo baby
* @return    Integer    Body count (but any less than 500 and it's not really worth mentioning)
*/
function rambo() {// Get the victims and initiate that body count status
    $victims = func_get_args();
    $body_count = 0;    // Kill those damn punks
    foreach($victims as $victim) {
        if($death_and_suffering = @unset($victim)) {
            $body_count++;
        }
    }// How many kills did Rambo tally up on this mission?
    return($body_count);
}
?>

Anonymous ¶

12 years ago

further I realized that an object, when getting detroyed, does care about destroying variable in object space visibility but not those in local visibility, be aware of the found pattern:

class release_test{
  private $buffer;
  private $other_object;
  public function __construct(){
    $this->other_object=new other_object_class();
  }
  public function __destruct(){
    //note that you always have to unset class objects, in order to get the resources released
    unset($this->other_object);
  }
  public allocate_mem_A(){
    $this->buffer=file("/tmp/bigfile");
  }
  public allocate_mem_B(){
    $buffer=file("/tmp/bigfile");
  }
  public allocate_mem_C(){
    $buffer=file("/tmp/bigfile");
    unset($buffer);
  }
  public allocate_mem_D(){
    $this->other_buffer=file("/tmp/bigfile");
  }
} //this does not lead to a resource problem
$A=new release_test();
$A->allocate_mem_A();
$A->__destruct();
unset($A); //this DOES lead to a resource problem
$B=new release_test();
$B->allocate_mem_B();
$B->__destruct();
unset($B); //this does not lead to a resource problem
$C=new release_test();
$C->allocate_mem_C();
$C->__destruct();
unset($C); //this does not lead to a resource problem
$D=new release_test();
$D->allocate_mem_D();
$D->__destruct();
unset($D);
?>

warhog at warhog dot net ¶

18 years ago

you may wan't to unset all variables which are defined, here's one way:

function unset_all_vars($a)
{ foreach($a as $key => $val)
  { unset($GLOBALS[$key]); }
  return serialize($a); }unset_all_vars(get_defined_vars());?>

you can also save than a serialized var of the "memory" and perhaps store this in a temporary file.. very usefull if you work with text files and/or file uploads when you've got very large variables.

greetz

dibakar dot datta at gmail dot com ¶

16 years ago

Instead of using the unset function  for unregistering your session or other array values you can also do this samll feature and get this task done with just 1 line code.

Suppose, if you like to unregister your session store values.
You can use:

      $_SESSION = array();

Well this syntax saves lot's of time instead of unsetting each values.

edouard dot berge at gmail dot com ¶

12 years ago

Warning!

When unset from an array, if you unset all elements, the array is always set

$tab=array('A'=>1,'B'=>2);
unset($tab['A']);
unset($tab['B']);
echo isset($tab)." ".count($tab);

output: 1 0

Hayley Watson ¶

15 years ago

In regard to some confusion earlier in these notes about what causes unset() to trigger notices when unsetting variables that don't exist....

Unsetting variables that don't exist, as in
unset($undefinedVariable);
?>
does not trigger an "Undefined variable" notice. But
unset($undefinedArray[$undefinedKey]);
?>
triggers two notices, because this code is for unsetting an element of an array; neither $undefinedArray nor $undefinedKey are themselves being unset, they're merely being used to locate what should be unset. After all, if they did exist, you'd still expect them to both be around afterwards. You would NOT want your entire array to disappear just because you unset() one of its elements!

stacionari at gmail dot com ¶

14 years ago

Sometimes you need to assigne values to an array index in some loop (if, while, foreach etc.) but you wish to set starting index key to some number greater then zero (lets say 5). One idea how to do this is:

    $values = array(5, 10, 15, 100);  //array of values that we wish to add to our new array$myArray = array(4=>0);   //sets starting key to be 4 and assigns some value (lets say 0)
    unset($myArray[4]);   //delete this index key, but preserves further enumeration foreach($values as $value){
        $myArray[] = $value;   //asign values to our array
    }print_r($myArray);/* Output:

Array ( [5] => 5 [6] => 10 [7] => 15 [8] => 100 )

*/

Andreas ¶

12 years ago

You can not unset a numeric key of an array, if key is a string. See this example:

// Create a simple array with 3 different key types
$test[1] = array(
    10 => array('apples'),
    "20" => array('bananas'),
    '30' => array('peaches')
);
$test[2] = (array) json_decode(json_encode($test[1]));
$test[3] = (array) (object) $test[1];
// array form a stdClass object
$testClass = new stdClass();
$testClass->{10} =  array('apples');
$testClass->{"20"} =  array('bananas');
$test[4] = (array) $testClass[6];

echo "

";
foreach($test as $testNum => $arr) {
    echo "\nTest: " . $testNum  . " \n";
    var_dump($arr);
    foreach($arr as $key => $fruit) {
        echo "key: " . $key . "\n";
        echo "key exists: ";
        var_dump(array_key_exists(strval($key), $arr));
        echo "typeof key is: " . gettype($key) . "\n";
        unset($arr[$key]);
    }
    var_dump($arr);
    echo "\n" . str_repeat("-", 80);
}
echo "

And here is the output:

Test: 1
array(3) {
  [10]=>
  array(1) {
    [0]=>
    string(6) "apples"
  }
  [20]=>
  array(1) {
    [0]=>
    string(7) "bananas"
  }
  [30]=>
  array(1) {
    [0]=>
    string(7) "peaches"
  }
}
key: 10
key exists: bool(true)
typeof key is: integer
key: 20
key exists: bool(true)
typeof key is: integer
key: 30
key exists: bool(true)
typeof key is: integer
array(0) {
}

--------------------------------------------------------------
Test: 2
array(3) {
  ["10"]=>
  array(1) {
    [0]=>
    string(6) "apples"
  }
  ["20"]=>
  array(1) {
    [0]=>
    string(7) "bananas"
  }
  ["30"]=>
  array(1) {
    [0]=>
    string(7) "peaches"
  }
}
key: 10
key exists: bool(false)
typeof key is: string
key: 20
key exists: bool(false)
typeof key is: string
key: 30
key exists: bool(false)
typeof key is: string
array(3) {
  ["10"]=>
  array(1) {
    [0]=>
    string(6) "apples"
  }
  ["20"]=>
  array(1) {
    [0]=>
    string(7) "bananas"
  }
  ["30"]=>
  array(1) {
    [0]=>
    string(7) "peaches"
  }
}

--------------------------------------------------------------
Test: 3
array(3) {
  [10]=>
  array(1) {
    [0]=>
    string(6) "apples"
  }
  [20]=>
  array(1) {
    [0]=>
    string(7) "bananas"
  }
  [30]=>
  array(1) {
    [0]=>
    string(7) "peaches"
  }
}
key: 10
key exists: bool(true)
typeof key is: integer
key: 20
key exists: bool(true)
typeof key is: integer
key: 30
key exists: bool(true)
typeof key is: integer
array(0) {
}

--------------------------------------------------------------
Test: 4
array(2) {
  ["10"]=>
  array(1) {
    [0]=>
    string(6) "apples"
  }
  ["20"]=>
  array(1) {
    [0]=>
    string(7) "bananas"
  }
}
key: 10
key exists: bool(false)
typeof key is: string
key: 20
key exists: bool(false)
typeof key is: string
array(2) {
  ["10"]=>
  array(1) {
    [0]=>
    string(6) "apples"
  }
  ["20"]=>
  array(1) {
    [0]=>
    string(7) "bananas"
  }
}

--------------------------------------------------------------

Fix the problem with a rebuild of the array:
$oldArray = $array();
$array = array();
foreach($oldArray as $key => $item) {
    $array[intval($key)] = $item;
}

macnimble at gmail dot com ¶

13 years ago

Two ways of unsetting values within an array:

# remove by key:
function array_remove_key ()
{
  $args  = func_get_args();
  return array_diff_key($args[0],array_flip(array_slice($args,1)));
}
# remove by value:
function array_remove_value ()
{
  $args = func_get_args();
  return array_diff($args[0],array_slice($args,1));
}$fruit_inventory = array(
  'apples' => 52,
  'bananas' => 78,
  'peaches' => 'out of season',
  'pears' => 'out of season',
  'oranges' => 'no longer sold',
  'carrots' => 15,
  'beets' => 15,
);

echo

Original Array:\n",
     print_r($fruit_inventory,TRUE),
     '

Array after key removal:\n",
     print_r($fruit_inventory,TRUE),
     '

Array after value removal:\n",
     print_r($fruit_inventory,TRUE),
     '

chad 0x40 herballure 0x2e com ¶

15 years ago

It is observed on PHP 5.1.6 that unset($this); ?> inside of a method will remove the reference to $this in that method. $this isn't considered "special" as far as unset() is concerned.

tigercat at aol dot com ¶

9 years ago

The combination of "global" and "unset" in functions can lead to some unexpected results.  This is because the "global" function creates a reference to a variable at the time it's executed, so a variable can be deleted out from under a "global $my_variable" declaration in a function.  Accessing data that's been deleted with an obsolete reference is usually a bad thing; in some languages it can generate a machine address fault.

$my_global_var = "old data";
f1();

function

muhamad_zakaria at yahoo dot com ¶

17 years ago

We have experienced when we applied 'unset' to the overloaded properties (PHP5), consider the code below:
    class TheObj {
        public $RealVar1, $RealVar2, $RealVar3, $RealVar4;
        public $Var = array();

        function

magnesium dot oxide dot play+php at gmail dot com ¶

8 years ago

You can unset superglobals like $GLOBALS, $_GET etc., but causing an unususal behavior (as of PHP 5.3.3).

1) unsetting of superglobals is done globally, i.e. unsetting inside the function affects GLOBALLY.

2) Recreation of unset'ed superglobals can be done (recreated valiables are superglobals), but original functionality (in $GLOBALS, $_SESSION ...) has lost.

function foo(){
  unset($GLOBALS);
}

function