今天整理代碼想使用$GLOBALS['_SERVER']來替代$_SERVER來訪問相關的環境變量，總是會報“_SERVER undefined”錯誤。如下用例： 用例1： <?php print_r($GLOBALS); 此時輸出中並沒有_SERVER相關信息： Array ( [GLOBALS] => Array *RECURSION* [_POST] => Array ( ) [_GET] => Array ( ) [_COOKIE] => Array ( ) [_FILES] => Array ( ) ) 用例2： <?php print_r($GLOBALS); print_r($_SERVER); 此時輸出中含有_SERVER相關信息： Array ( [GLOBALS] => Array *RECURSION* [_POST] => Array ( ) [_GET] => Array ( ) [_COOKIE] => Array ( ) [_FILES] => Array ( ) [_SERVER] => Array ( ) ) 查了下PHP手冊關於$GLOBALS描述，引用therandshow at gmail dot com的評論： 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. 追根數源，發現PHP5Changelog更新日志的描述： Unordered List ItemImproved Zend Engine, performance tweaks and optimizations Unordered List ItemChanged $GLOBALS into a JIT autoglobal, so it's initialized only if used. (this may affect opcode caches!)www.2cto.com 718 ; When enabled, the SERVER and ENV variables are created when they're first 719 ; used (Just In Time) instead of when the script starts. If these variables 720 ; are not used within a script, having this directive on will result in a 721 ; performance gain. The PHP directives register_globals, register_long_arrays, 722 ; and register_argc_argv must be disabled for this directive to have any affect. 723 ; http://php.net/auto-globals-jit 724 auto_globals_jit = On 終於弄明白了，PHP5+中在開啟auto_globals_jit = On情況下，$_SERVER變量和$_ENV變量不會在腳本啟動時就創建，而是會在第一次使用$SERVER和$ENV時才會創建。所以就會出現上述兩個用例的情況。 備注： 實測結論： auto_globals_jit setting is also affecting $_REQUEST superglobal in 5.3 It is not explicitly stated in documentation. 至少5.3.13版本中開啟auto_globals_jit = On情況下，$_REQUEST也只會在第一次使用時才會創建。
