实例：

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

2，http://localhost/aaa/?p=222 (附带查询)
结果：
$_SERVER['QUERY_STRING'] = "p=222";
$_SERVER['REQUEST_URI']  = "/aaa/?p=222";
$_SERVER['SCRIPT_NAME']  = "/aaa/index.php";
$_SERVER['PHP_SELF']     = "/aaa/index.php";

3，http://localhost/aaa/index.php?p=222&q=333
结果：
$_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";

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