php get user input for iframe id

How do I make the present id (wfn-374 ) a variable that I can get from a user input form on same or higher page? New to this and amateur. Thank you!
[CODE]
<IFRAME FRAMEBORDER="0" SCROLLING="NO" MARGINWIDTH="0" MARGINHEIGHT="0" WIDTH="960" HEIGHT="506" src="http://watchitoo.com/iframe.php?id=wfn-374&scale=false&layout=15&wl=no_logos:true ">Browser does not support iframes!</IFRAME>
*[CODE]

Thanks all, now answered by Old Pedant - works perfectly.

Check the below link i think it will help you out
http://stackoverflow.com/questions/10024577/get-variable-from-iframe-after-form-is-submit-to-it