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PHP unset()函数是用来销毁变量的，但很多时候，这个函数只把变量给销毁了，内存中存放的该变量的值仍然没有销毁，也就是没能达到我们想要的释放内存的效果。这里我建议大家用 $变量=null 的方法来释放其内存。原因看了下面的就知道了。
以下是关于PHP unset()函数的几点要点：（以下均在windows环境下测试，php 2.5.9）
1. 该函数只有在变量值所占空间超过256字节长的时候才会释放内存
2. 只有当指向该值的所有变量（比如有引用变量指向该值）都被销毁后，地址才会被释放（也要执行1的判断）
下面给出例子代码论证：
<ol class="dp-xml">
<li class="alt"><span><span class="tag"><span> ?php </span></span></span></li>
<li>
<span>$</span><span class="attribute">test</span><span>=</span><span class="attribute-value">str_repeat</span><span>("1",256); </span>
</li>
<li class="alt">
<span>$</span><span class="attribute">s</span><span> = </span><span class="attribute-value">memory_get_usage</span><span>(); </span>
</li>
<li><span>//改函数用来查看当前所用内存 </span></li>
<li class="alt"><span>unset($test); </span></li>
<li>
<span>$</span><span class="attribute">e</span><span> = </span><span class="attribute-value">memory_get_usage</span><span>(); </span>
</li>
<li class="alt"><span>echo ' 释放内存: '.($s-$e); </span></li>
<li>
<span>//输出为272，但如果上面test变量改为<br>$</span><span class="attribute">test</span><span>=</span><span class="attribute-value">str_repeat</span><span>("1",255)，输出则为0 </span>
</li>
<li class="alt">
<span class="tag">?></span><span> </span>
</li>
</ol>登录后复制
至于为什么是272而不是256，就不是很清楚了，不知道内部是怎么处理的。
<ol class="dp-xml">
<li class="alt"><span><span class="tag"><span> ?php </span></span></span></li>
<li>
<span>$</span><span class="attribute">test</span><span> = </span><span class="attribute-value">str_repeat</span><span>("1",256); </span>
</li>
<li class="alt">
<span>$</span><span class="attribute">p</span><span> = &$test; </span>
</li>
<li><span>unset($test); </span></li>
<li class="alt"><span>echo $p; </span></li>
<li><span>//输出为256个1。如果上面改为unset($p)<br>，更不行了，echo $test 直接显示为256个1 </span></li>
<li class="alt">
<span class="tag">?></span><span> </span>
</li>
</ol>登录后复制
也就是说内存中赋给$a的值仍然存在。可见unset()并没达到释放内存的效果。
但如果在上述代码中加入$test=null,或者再加一个unset($p),就能达到释放内存效果了，PHP unset()函数测试代码如下：
变量赋值为null方法：
<ol class="dp-xml">
<li class="alt"><span><span class="tag"><span> ?php </span></span></span></li>
<li>
<span>$</span><span class="attribute">test</span><span> = </span><span class="attribute-value">str_repeat</span><span>("1",256); </span>
</li>
<li class="alt">
<span>$</span><span class="attribute">p</span><span> = &$test; </span>
</li>
<li>
<span>$</span><span class="attribute">s</span><span> = </span><span class="attribute-value">memory_get_usage</span><span>(); </span>
</li>
<li class="alt">
<span>$</span><span class="attribute">test</span><span> = </span><span class="attribute-value">null</span><span>; </span>
</li>
<li><span>unset($test); </span></li>
<li class="alt">
<span>$</span><span class="attribute">e</span><span> = </span><span class="attribute-value">memory_get_usage</span><span>(); </span>
</li>
<li><span>echo ' 释放内存: '.($s-$e); </span></li>
<li><span>//输出为272 </span></li>
<li class="alt"><span>var_dump($p); //输出为NULL </span></li>
<li>
<span class="tag">?></span><span> </span>
</li>
</ol>登录后复制
将指向该地址中值的变量全部销毁的方法：
<ol class="dp-xml">
<li class="alt"><span><span class="tag"><span> ?php </span></span></span></li>
<li>
<span>$</span><span class="attribute">test</span><span> = </span><span class="attribute-value">str_repeat</span><span>("1",256); </span>
</li>
<li class="alt">
<span>$</span><span class="attribute">p</span><span> = &$test; </span>
</li>
<li>
<span>$</span><span class="attribute">s</span><span> = </span><span class="attribute-value">memory_get_usage</span><span>(); </span>
</li>
<li class="alt"><span>//注意，以下2个unset()顺序对调没<br>有关系，不影响结果 </span></li>
<li><span>unset($p); </span></li>
<li class="alt"><span>unset($test); </span></li>
<li>
<span>$</span><span class="attribute">e</span><span> = </span><span class="attribute-value">memory_get_usage</span><span>(); </span>
</li>
<li class="alt"><span>echo ' 释放内存: '.($s-$e); //输出为272 </span></li>
<li>
<span class="tag">?></span><span> </span>
</li>
</ol>登录后复制
到此PHP unset()函数论证完毕。
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