How to destroy unset in php_PHP tutorial-PHP Tutorial-php.cn

How to destroy unset in php_PHP tutorial

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Release： 2016-07-20 11:02:49

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php tutorial variable destruction and usage of unset

unset -- Release the given variable
Description
void unset (mixed var [, mixed var [, ...]])

unset() destroys the specified variable. Note that in PHP 3, unset() will return TRUE (actually the integer value 1), while in PHP 4, unset() is no longer a real function: it is now a statement. There is no return value, and trying to get the return value of unset() will result in a parsing error.

Refer to php manual:

/* 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教程.

如此便能够说明php 的 unset是如何进行的

The first thing to emphasize is that unset is no longer a function in PHP. Since it is not a function, there is no return value, so the return value of unset cannot be used to make judgments when using it.

Secondly, in a function, unset can only destroy local variables, but not global variables. Let’s take a look at an example in the manual

function destroy_foo() {
global $foo;
unset($foo);
}

$foo = ‘bar’;
destroy_foo();
echo $foo;
?>

The result returned by

bar

http://www.bkjia.com/phper/php/37201.htm