PHP5+引进$GLOBALS延迟初始化的概念
用例1：
print_r($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 $GLOBALS into a JIT autoglobal, so it's initialized only if used. (this may affect opcode caches!)www.2cto.com
718 ; When enabled, the SERVER and ENV variables are created when they're first
719 ; used (Just In Time) instead of when the script starts. If these variables
720 ; are not used within a script, having this directive on will result in a
721 ; performance gain. The PHP directives register_globals, register_long_arrays,
722 ; and register_argc_argv must be disabled for this directive to have any affect.
723 ; http://php.net/auto-globals-jit
724 auto_globals_jit = On
终于弄明白了，PHP5+中在开启auto_globals_jit = On情况下，$_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。