PHP5+引进$GLOBALS推迟初始化的概念

PHP5+引进$GLOBALS延迟初始化的概念

  作者:zhanhailiang 日期:2013-01-18

今天整理代码想使用$GLOBALS['_SERVER']来替代$_SERVER来访问相关的环境变量，总是会报“_SERVER undefined”错误。如下用例：

用例1：

<span class="kw2" style="color:rgb(0,0,0); font-weight:bold"><?php</span>
 
<span class="kw3" style="color:rgb(0,0,102)">print_r</span><span class="br0" style="color:rgb(102,204,102)">(</span><span class="re0" style="color:rgb(0,0,255)">$GLOBALS</span><span class="br0" style="color:rgb(102,204,102)">)</span><span class="sy0" style="color:rgb(102,204,102)">;</span>

此时输出中并没有_SERVER相关信息：

<span class="kw3" style="color:rgb(0,0,102)">Array</span>
<span class="br0" style="color:rgb(102,204,102)">(</span>
    <span class="br0" style="color:rgb(102,204,102)">[</span>GLOBALS<span class="br0" style="color:rgb(102,204,102)">]</span> <span class="sy0" style="color:rgb(102,204,102)">=></span> <span class="kw3" style="color:rgb(0,0,102)">Array</span>
 <span class="sy0" style="color:rgb(102,204,102)">*</span>RECURSION<span class="sy0" style="color:rgb(102,204,102)">*</span>
    <span class="br0" style="color:rgb(102,204,102)">[</span>_POST<span class="br0" style="color:rgb(102,204,102)">]</span> <span class="sy0" style="color:rgb(102,204,102)">=></span> <span class="kw3" style="color:rgb(0,0,102)">Array</span>
        <span class="br0" style="color:rgb(102,204,102)">(</span>
        <span class="br0" style="color:rgb(102,204,102)">)</span>
 
    <span class="br0" style="color:rgb(102,204,102)">[</span>_GET<span class="br0" style="color:rgb(102,204,102)">]</span> <span class="sy0" style="color:rgb(102,204,102)">=></span> <span class="kw3" style="color:rgb(0,0,102)">Array</span>
        <span class="br0" style="color:rgb(102,204,102)">(</span>
        <span class="br0" style="color:rgb(102,204,102)">)</span>
 
    <span class="br0" style="color:rgb(102,204,102)">[</span>_COOKIE<span class="br0" style="color:rgb(102,204,102)">]</span> <span class="sy0" style="color:rgb(102,204,102)">=></span> <span class="kw3" style="color:rgb(0,0,102)">Array</span>
        <span class="br0" style="color:rgb(102,204,102)">(</span>
        <span class="br0" style="color:rgb(102,204,102)">)</span>
 
    <span class="br0" style="color:rgb(102,204,102)">[</span>_FILES<span class="br0" style="color:rgb(102,204,102)">]</span> <span class="sy0" style="color:rgb(102,204,102)">=></span> <span class="kw3" style="color:rgb(0,0,102)">Array</span>
        <span class="br0" style="color:rgb(102,204,102)">(</span>
        <span class="br0" style="color:rgb(102,204,102)">)</span>
<span class="br0" style="color:rgb(102,204,102)">)</span>

用例2：

<span class="kw2" style="color:rgb(0,0,0); font-weight:bold"><?php</span><span style="color:#333333">
 
</span><span class="kw3" style="color:rgb(0,0,102)">print_r</span><span class="br0" style="color:rgb(102,204,102)">(</span><span class="re0" style="color:rgb(0,0,255)">$GLOBALS</span><span class="br0" style="color:rgb(102,204,102)">)</span><span class="sy0" style="color:rgb(102,204,102)">;</span><span style="color:#333333">
</span><span style="color:#ff0000"><span class="kw3">print_r</span><span class="br0">(</span><span class="re0">$_SERVER</span><span class="br0">)</span><span class="sy0">;</span></span>

此时输出中含有_SERVER相关信息：

<span class="kw3" style="color:rgb(0,0,102)">Array</span><span style="color:#333333">
</span><span class="br0" style="color:rgb(102,204,102)">(</span><span style="color:#333333">
    </span><span class="br0" style="color:rgb(102,204,102)">[</span><span style="color:#333333">GLOBALS</span><span class="br0" style="color:rgb(102,204,102)">]</span><span style="color:#333333"> </span><span class="sy0" style="color:rgb(102,204,102)">=></span><span style="color:#333333"> </span><span class="kw3" style="color:rgb(0,0,102)">Array</span><span style="color:#333333">
 </span><span class="sy0" style="color:rgb(102,204,102)">*</span><span style="color:#333333">RECURSION</span><span class="sy0" style="color:rgb(102,204,102)">*</span><span style="color:#333333">
    </span><span class="br0" style="color:rgb(102,204,102)">[</span><span style="color:#333333">_POST</span><span class="br0" style="color:rgb(102,204,102)">]</span><span style="color:#333333"> </span><span class="sy0" style="color:rgb(102,204,102)">=></span><span style="color:#333333"> </span><span class="kw3" style="color:rgb(0,0,102)">Array</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">(</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">)</span><span style="color:#333333">
 
    </span><span class="br0" style="color:rgb(102,204,102)">[</span><span style="color:#333333">_GET</span><span class="br0" style="color:rgb(102,204,102)">]</span><span style="color:#333333"> </span><span class="sy0" style="color:rgb(102,204,102)">=></span><span style="color:#333333"> </span><span class="kw3" style="color:rgb(0,0,102)">Array</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">(</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">)</span><span style="color:#333333">
 
    </span><span class="br0" style="color:rgb(102,204,102)">[</span><span style="color:#333333">_COOKIE</span><span class="br0" style="color:rgb(102,204,102)">]</span><span style="color:#333333"> </span><span class="sy0" style="color:rgb(102,204,102)">=></span><span style="color:#333333"> </span><span class="kw3" style="color:rgb(0,0,102)">Array</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">(</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">)</span><span style="color:#333333">
 
    </span><span class="br0" style="color:rgb(102,204,102)">[</span><span style="color:#333333">_FILES</span><span class="br0" style="color:rgb(102,204,102)">]</span><span style="color:#333333"> </span><span class="sy0" style="color:rgb(102,204,102)">=></span><span style="color:#333333"> </span><span class="kw3" style="color:rgb(0,0,102)">Array</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">(</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">)</span><span style="color:#333333">
 
    </span><span class="br0" style="color:rgb(102,204,102)">[</span><span style="color:#ff0000">_SERVER</span><span class="br0" style="color:rgb(102,204,102)">]</span><span style="color:#333333"> </span><span class="sy0" style="color:rgb(102,204,102)">=></span><span style="color:#333333"> </span><span class="kw3" style="color:rgb(0,0,102)">Array</span><span style="color:#333333">
        </span><span class="br0" style="color:rgb(102,204,102)">(</span><span style="color:#333333">
 
        </span><span class="br0" style="color:rgb(102,204,102)">)</span><span style="color:#333333">
 
</span><span class="br0" style="color:rgb(102,204,102)">)</span>

查了下PHP手册关于$GLOBALS描述，引用therandshow at gmail dot com的评论：

therandshow at gmail dot com

As of PHP 5.4 $GLOBALS is now initialized just-in-time. This means there now is an advantage to not use 
the $GLOBALS variable as you can avoid the overhead of initializing it. How much of an advantage that is 
I'm not sure, but I've never liked $GLOBALS much anyways.

追根数源，发现PHP5Changelog更新日志的描述：

Unordered List ItemImproved Zend Engine, performance tweaks and optimizations

Unordered List ItemChanged $GLOBALS into a JIT autoglobal, so it's initialized only if used. (this may affect opcode caches!)

<span style="color:#333333"> </span><span class="nu0" style="color:rgb(204,102,204)">718</span><span style="color:#333333"> </span><span class="co0" style="color:rgb(128,128,128); font-style:italic">; </span><span class="co0" style="font-style:italic"><span style="color:#ff0000">When enabled, the SERVER and ENV variables are created when they're first</span></span><span style="color:#333333">
 </span><span class="nu0" style="color:rgb(204,102,204)">719</span><span style="color:#333333"> </span><span class="co0" style="color:rgb(128,128,128); font-style:italic">; </span><span class="co0" style="font-style:italic"><span style="color:#ff0000">used (Just In Time) instead of when the script starts.</span></span><span class="co0" style="color:rgb(128,128,128); font-style:italic"> If these variables</span><span style="color:#333333">
 </span><span class="nu0" style="color:rgb(204,102,204)">720</span><span style="color:#333333"> </span><span class="co0" style="color:rgb(128,128,128); font-style:italic">; are not used within a script, having this directive on will result in a</span><span style="color:#333333">
 </span><span class="nu0" style="color:rgb(204,102,204)">721</span><span style="color:#333333"> </span><span class="co0" style="color:rgb(128,128,128); font-style:italic">; performance gain. The PHP directives register_globals, register_long_arrays,</span><span style="color:#333333">
 </span><span class="nu0" style="color:rgb(204,102,204)">722</span><span style="color:#333333"> </span><span class="co0" style="color:rgb(128,128,128); font-style:italic">; and register_argc_argv must be disabled for this directive to have any affect.</span><span style="color:#333333">
 </span><span class="nu0" style="color:rgb(204,102,204)">723</span><span style="color:#333333"> </span><span class="co0" style="color:rgb(128,128,128); font-style:italic">; http://php.net/auto-globals-jit</span><span style="color:#333333">
 </span><span class="nu0" style="color:rgb(204,102,204)">724</span><span style="color:#333333"> auto_globals_jit </span><span class="sy0" style="color:rgb(102,204,102)">=</span><span class="re2" style="color:rgb(0,0,255)"> On</span>

终于弄明白了，PHP5+中在开启auto_globals_jit = On情况下，$_SERVER变量和$_ENV变量不会在脚本启动时就创建，而是会在第一次使用$SERVER和$ENV时才会创建。所以就会出现上述两个用例的情况。

备注：

实测结论：

auto_globals_jit setting is also affecting $_REQUEST superglobal in 5.3 It is not explicitly stated in documentation. 
至少5.3.13版本中开启auto_globals_jit = On情况下，$_REQUEST也只会在第一次使用时才会创建。