php教程

取消引用

当 unset 一个引用,只是断开了变量名和变量内容之间的绑定。这并不意味着变量内容被销毁了。例如:

<?php
$a 
1;
$b =& $a;
unset(
$a);
?>
不会 unset $b,只是 $a

再拿这个和 Unix 的 unlink 调用来类比一下可能有助于理解。

User Contributed Notes

ojars26 at NOSPAM dot inbox dot lv 03-May-2008 07:41
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.
sony-santos at bol dot com dot br 05-Feb-2007 03:56
<?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.
lazer_erazer 05-Sep-2006 04:02
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 07:10
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!
?>
libi 24-Jan-2006 12:20
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.

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