php变量销毁unset的用法_PHP教程

内容导读

互联网集市收集整理的这篇技术教程文章主要介绍了php变量销毁unset的用法_PHP教程，小编现在分享给大家，供广大互联网技能从业者学习和参考。文章包含4569字，纯文字阅读大概需要7分钟。

内容图文

unset -- 释放给定的变量
描述
void unset ( mixed var [, mixed var [, ...]])

unset() 销毁指定的变量。注意在 PHP 3 中，unset() 将返回 TRUE（实际上是整型值 1），而在 PHP 4 中，unset() 不再是一个真正的函数：它现在是一个语句。这样就没有了返回值，试图获取 unset() 的返回值将导致解析错误。

参考php手册：

/* 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

先要强调的一点是unset在php中已经不再是一个函数了，既然不是函数，那么就没有了返回值，所以用的时候不能够用unset的返回值来做判断。

其次，在函数中，unset只能销毁局部变量，并不能销毁全局变量，来看下手册的一个例子

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

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

返回的结果为

bar

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

http://www.bkjia.com/PHPjc/445335.htmlwww.bkjia.comtruehttp://www.bkjia.com/PHPjc/445335.htmlTechArticlephp教程变量销毁unset的用法 unset -- 释放给定的变量 描述 void unset ( mixed var [, mixed var [, ...]]) unset() 销毁指定的变量。注意在 PHP 3 中，unset(...

内容总结

以上是互联网集市为您收集整理的php变量销毁unset的用法_PHP教程全部内容，希望文章能够帮你解决php变量销毁unset的用法_PHP教程所遇到的程序开发问题。 如果觉得互联网集市技术教程内容还不错，欢迎将互联网集市网站推荐给程序员好友。

内容备注

版权声明：本文内容由互联网用户自发贡献，该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务，不拥有所有权，不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容， 请发送邮件至 gblab@vip.qq.com 举报，一经查实，本站将立刻删除。

内容手机端