实例：
1、http://localhost/aaa/ （打开aaa中的index.php）
$_SERVER['QUERY_STRING'] = "";
$_SERVER['REQUEST_URI']  = "/aaa/";
$_SERVER['SCRIPT_NAME']  = "/aaa/index.php";
$_SERVER['PHP_SELF']     = "/aaa/index.php";

2、http://localhost/aaa/index.php?p=222&q=333（带参数）
$_SERVER['QUERY_STRING'] = "p=222&q=333";
$_SERVER['REQUEST_URI']  = "/aaa/index.php?p=222&q=333";
$_SERVER['SCRIPT_NAME']  = "/aaa/index.php";
$_SERVER['PHP_SELF']     = "/aaa/index.php";

3、http://localhost/tag/test  →  http://localhost/index.php?m=tag&tag=test（URL重写）
$_SERVER['QUERY_STRING'] = "m=tag&tag=test";
$_SERVER['REQUEST_URI']  = "/tag/test";
$_SERVER['SCRIPT_NAME']  = "/index.php";
$_SERVER['PHP_SELF']     = "/index.php";

由实例可知：
$_SERVER["QUERY_STRING"]  获取查询 语句，实例中可知，获取的是?后面的值
$_SERVER["REQUEST_URI"]    获取 http://localhost 后面的值，包括/
$_SERVER["SCRIPT_NAME"]    获取当前脚本的路径，如：index.php
$_SERVER["PHP_SELF"]           当前正在执行脚本的文件名