Passing URL Variables (EASY!)

I have a url that I need to grab a variable from that is coming from an outside website. The url look like this. eg. http://www.website.com/id_sub=(This Variable) The variable above will be enter on the fly by the site above. The traffic from that url is then passed to my site, I need to grab that variable on my first page from the url and then pass it to the second page. This is a very simple project and I need to have it done by today. Please message me if you have any other question. Happy Bidding!