PHP5+引进$GLOBALS推迟初始化的概念

  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：

 
print_r($GLOBALS);

此时输出中并没有_SERVER相关信息：

Array
(
    [GLOBALS] => Array
 *RECURSION*
    [_POST] => Array
        (
        )
 
    [_GET] => Array
        (
        )
 
    [_COOKIE] => Array
        (
        )
 
    [_FILES] => Array
        (
        )
)

用例2：

 
print_r($GLOBALS);
print_r($_SERVER);

此时输出中含有_SERVER相关信息：

Array
(
    [GLOBALS] => Array
 *RECURSION*
    [_POST] => Array
        (
        )
 
    [_GET] => Array
        (
        )
 
    [_COOKIE] => Array
        (
        )
 
    [_FILES] => Array
        (
        )
 
    [_SERVER] => Array
        (
 
        )
 
)

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!)

 718 ; When enabled, the SERVER and ENV variables are created when they're first
 719 ; used (Just In Time) instead of when the script starts. If these variables
 720 ; are not used within a script, having this directive on will result in a
 721 ; performance gain. The PHP directives register_globals, register_long_arrays,
 722 ; and register_argc_argv must be disabled for this directive to have any affect.
 723 ; http://php.net/auto-globals-jit
 724 auto_globals_jit = On

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。