认识预定义变量

$GLOBALS — 引用全局作用域中可用的全部变量

$_SERVER

$_GET

$_POST

$_FILES

$_REQUEST 默认情况下包含了 $_GET，$_POST和 $_COOKIE 的数组。 

$_ENV

$_SESSION

session_start();

session_id — 获取/设置当前会话 ID

session_write_close — Write session data and end session

$_COOKIE

setcookie ($name ,$value,$expire,$path, $domain,$secure,$httponly)