Check if PHP session has already started

I have a PHP file that is sometimes called from a page that has started a session and sometimes from a page that doesn't have session started. Therefore when I have `session_start()` on this script I sometimes get the error message for "session already started". For that I've put these lines: if(!isset($_COOKIE["PHPSESSID"])) { session_start(); } but this time I got this warning message: > Notice: Undefined variable: _SESSION Is there a better way to check if session has already started? If I use `@session_start` will it make things work properly and just shut up the warnings?