PHP: Unsetting References

Unsetting References

When you unset the reference, you just break the binding between variable name and variable content. This does not mean that variable content will be destroyed. For example:

<?php

$a = 1;
$b =& $a;
unset($a); 

?>

won't unset $b, just $a.

Again, it might be useful to think about this as analogous to the Unix unlink call.

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User Contributed Notes 7 notes

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442

ojars26 at NOSPAM dot inbox dot lv ¶

16 years ago

Simple look how PHP Reference works
<?php
/* Imagine this is memory map
 ______________________________
|pointer | value | variable              |
 -----------------------------------
|   1     |  NULL  |         ---           |
|   2     |  NULL  |         ---           |
|   3     |  NULL  |         ---           |
|   4     |  NULL  |         ---           |
|   5     |  NULL  |         ---           |
------------------------------------
Create some variables   */
$a=10;
$b=20;
$c=array ('one'=>array (1, 2, 3));
/* Look at memory
 _______________________________
|pointer | value |       variable's       |
 -----------------------------------
|   1     |  10     |       $a               |
|   2     |  20     |       $b               |
|   3     |  1       |      $c['one'][0]   |
|   4     |  2       |      $c['one'][1]   |
|   5     |  3       |      $c['one'][2]   |
------------------------------------
do  */
$a=&$c['one'][2];
/* Look at memory
 _______________________________
|pointer | value |       variable's       |
 -----------------------------------
|   1     |  NULL  |       ---              |  //value of  $a is destroyed and pointer is free
|   2     |  20     |       $b               |
|   3     |  1       |      $c['one'][0]   |
|   4     |  2       |      $c['one'][1]   |
|   5     |  3       |  $c['one'][2]  ,$a | // $a is now here
------------------------------------
do  */
$b=&$a;  // or  $b=&$c['one'][2]; result is same as both "$c['one'][2]" and "$a" is at same pointer.
/* Look at memory
 _________________________________
|pointer | value |       variable's           |
 --------------------------------------
|   1     |  NULL  |       ---                  |  
|   2     |  NULL  |       ---                  |  //value of  $b is destroyed and pointer is free
|   3     |  1       |      $c['one'][0]       |
|   4     |  2       |      $c['one'][1]       |
|   5     |  3       |$c['one'][2]  ,$a , $b |  // $b is now here
---------------------------------------
next do */
unset($c['one'][2]);
/* Look at memory
 _________________________________
|pointer | value |       variable's           |
 --------------------------------------
|   1     |  NULL  |       ---                  |  
|   2     |  NULL  |       ---                  |  
|   3     |  1       |      $c['one'][0]       |
|   4     |  2       |      $c['one'][1]       |
|   5     |  3       |      $a , $b              | // $c['one'][2]  is  destroyed not in memory, not in array
---------------------------------------
next do   */
$c['one'][2]=500;    //now it is in array
/* Look at memory
 _________________________________
|pointer | value |       variable's           |
 --------------------------------------
|   1     |  500    |      $c['one'][2]       |  //created it lands on any(next) free pointer in memory
|   2     |  NULL  |       ---                  |  
|   3     |  1       |      $c['one'][0]       |
|   4     |  2       |      $c['one'][1]       |
|   5     |  3       |      $a , $b              | //this pointer is in use
---------------------------------------
lets tray to return $c['one'][2] at old pointer an remove reference $a,$b.  */
$c['one'][2]=&$a;
unset($a);
unset($b);   
/* look at memory
 _________________________________
|pointer | value |       variable's           |
 --------------------------------------
|   1     |  NULL  |       ---                  |  
|   2     |  NULL  |       ---                  |  
|   3     |  1       |      $c['one'][0]       |
|   4     |  2       |      $c['one'][1]       |
|   5     |  3       |      $c['one'][2]       | //$c['one'][2] is returned, $a,$b is destroyed
--------------------------------------- ?>
I hope this helps.

down

sony-santos at bol dot com dot br ¶

17 years ago

<?php
//if you do:

$a = "hihaha";
$b = &$a;
$c = "eita";
$b = $c;
echo $a; // shows "eita"

$a = "hihaha";
$b = &$a;
$c = "eita";
$b = &$c;
echo $a; // shows "hihaha"

$a = "hihaha";
$b = &$a;
$b = null;
echo $a; // shows nothing (both are set to null)

$a = "hihaha";
$b = &$a;
unset($b);
echo $a; // shows "hihaha"

$a = "hihaha";
$b = &$a;
$c = "eita";
$a = $c;
echo $b; // shows "eita"

$a = "hihaha";
$b = &$a;
$c = "eita";
$a = &$c;
echo $b; // shows "hihaha"

$a = "hihaha";
$b = &$a;
$a = null;
echo $b; // shows nothing (both are set to null)

$a = "hihaha";
$b = &$a;
unset($a);
echo $b; // shows "hihaha"
?>

I tested each case individually on PHP 4.3.10.

down

donny at semeleer dot nl ¶

18 years ago

Here's an example of unsetting a reference without losing an ealier set reference

<?php
$foo = 'Bob';              // Assign the value 'Bob' to $foo
$bar = &$foo;              // Reference $foo via $bar.
$bar = "My name is $bar";  // Alter $bar...
echo $bar;
echo $foo;                 // $foo is altered too.
$foo = "I am Frank";       // Alter $foo and $bar because of the reference
echo $bar;                 // output: I am Frank
echo $foo;                 // output: I am Frank

$foobar = &$bar;           // create a new reference between $foobar and $bar
$foobar = "hello $foobar"; // alter $foobar and with that $bar and $foo
echo $foobar;              //output : hello I am Frank
unset($bar);               // unset $bar and destroy the reference
$bar = "dude!";            // assign $bar
/* even though the reference between $bar and $foo is destroyed, and also the 
reference between $bar and $foobar is destroyed, there is still a reference 
between $foo and $foobar. */
echo $foo;                 // output : hello I am Frank
echo $bar;                 // output : due!
?>

down

lazer_erazer ¶

18 years ago

Your idea about unsetting all referenced variables at once is right,
just a tiny note that you changed NULL with unset()...
again, unset affects only one name and NULL affects the data,
which is kept by all the three names...

<?php
$a = 1;
$b =& $a;
$b = NULL; 
?>

This does also work!

<?php
$a = 1;
$b =& $a;
$c =& $b;
$b = NULL; 
?>

down

smcbride at msn dot com ¶

2 years ago

A little quirk on unset() when using references that may help someone.

If you want to delete the element of a reference to an array, you need to have the reference point to the parent of the key that you want to delete.

<?php
$arr = array('foo' => array('foo_sub1' => 'hey', 'foo_sub2' => 'you'), 'bar' => array('bar_sub1' => 'good', 'bar_sub2' => 'bye'));

$parref =  &$arr['foo'];
$childref = &$parref['foo_sub1'];

unset($childref);                   // this will simply unset the reference to child
unset($parref['foo_sub1']);   // this will actually unset the data in $arr;
$parref['foo_sub1'] = NULL;  // this will set the element to NULL, but not delete it.  If you run it after unset(), it add the key back and set it to NULL

?>

This is nice to use for passing something dynamically to a function by reference without copying the entire array to the function, but you want to do some maintenance on the array.

down

frowa at foxmail dot com ¶

3 years ago

it's my way to remember.

<?php

// the var $a is point to the value 1, as a line connect to value 1
$a = 1;      

// the var $b point to the value which the var $a point to, as a new line connect to value 1
$b =& $a;  

// cut the line of the var $a to value 1,now $a is freedom,it's nothing point to. so the value of $a is null
unset($a);
?>

         $a--------> 1
                           ↑
                            |
                            |
                           $b

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-3

libi ¶

18 years ago

clerca at inp-net dot eu dot org
"
If you have a lot of references linked to the same contents, maybe it could be useful to do this : 
<?php
$a = 1;
$b = & $a;
$c = & $b; // $a, $b, $c reference the same content '1'

$b = NULL; // All variables $a, $b or $c are unset
?>

"

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

NULL will not result in unseting the variables.
Its only change the value to "null" for all the variables.
becouse they all points to the same "part" in the memory.

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