Мне нужно знать, не пришел ли человек, который зашел на мой сайт, с другого конкретного сайта. Пример:

User A visits www.youtube.com/myvideo and clicks on a link to my website. User B visits google, search my website and click the link.

Сообщение о результатах на моей странице:

User A: Welcome! You already know how it works, register now! User B: Welcome! Please watch our video first at www.youtube.com/myvideo

Мой вопрос:

Is it possible to know the last url the user visited before entering my page?
I already tried $_SERVER['HTTP_REFERER'] with PHP, but it's not working correctly and I read that does not work on all browsers. Is there any language or php script to get this url, working on all browser correctly?