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$_server来访问相关的环境变量，总是会报“_server undefined”错误。如下用例：
用例1：
array *recursion* [_post] => array ( ) [_get] => array ( ) [_cookie] => array ( ) [_files] => array ( ))
用例2：
array *recursion* [_post] => array ( ) [_get] => array ( ) [_cookie] => array ( ) [_files] => array ( ) [_server] => array ( ) )
查了下php手册关于$globals描述，引用therandshow at gmail dot com的评论：
therandshow at gmail dot comas 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 optimizationsunordered 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。