Returning References
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.
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);
?>
Again, it might be useful to think about this as analogous to Unix unlink call.
add a note
User Contributed NotesUnsetting References
ojars26 at NOSPAM dot inbox dot lv
03-May-2008 10:41
03-May-2008 10:41
sony-santos at bol dot com dot br
05-Feb-2007 07:56
05-Feb-2007 07:56
<?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 $b; // shows "hihaha"
?>
I tested each case individually on PHP 4.3.10.
martin
05-Jan-2007 08:25
05-Jan-2007 08:25
note that in the previous example all variables (or the one data item all variables point to) is set to NULL, what is interpreted as !isset(), but the linkage between the variables still exists, so
<?php
echo (isset($a)?"set":"unset")."\n";
$a=1;
$b =& $a;
echo (isset($b)?"set":"unset")."\n";
$a=null;
echo (isset($b)?"set":"unset")."\n";
$a=1;
echo (isset($b)?"set":"unset")."\n";
?>
shows:
unset
set
unset
set
note that $b ist set again.
So if you want to brake the linkage, you have to use unset()
lazer_erazer
06-Sep-2006 07:02
06-Sep-2006 07:02
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;
?>
donny at semeleer dot nl
13-Jul-2006 10:10
13-Jul-2006 10:10
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!
?>
rustin dot phares at hotmail dot com
13-Mar-2006 02:31
13-Mar-2006 02:31
If you wish to unset both variables, you will need to unset the last referenced variable of that condition.
<?php
$a = 1;
$b =& $a;
unset($b);
?>
* These must be in a reference->copy hierarchy in order to unset; example:
<?php
$a = 1;
$b =& $a;
$c =& $b;
unset($c);
?>
This will not work:
<?php
$a = 1;
$b =& $a;
$c = $b;
unset($c);
?>
* Only $c is unset in the above example, meaning that both variables $b and $a are still assigned "1".
libi
24-Jan-2006 04:20
24-Jan-2006 04:20
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.
clerca at inp-net dot eu dot org
26-Nov-2005 04:33
26-Nov-2005 04:33
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
?>
I haven't test this trick a lot, but well, it seems to work greatly.