PHP cant take a variable from JavaScript, they process at different times, here is what happens under the hood.

Browser makes a request -> Server picks up request -> Server does all server side scripting (php, asp, sql, ect.) -> Server sends processed code back -> Browser gets it -> Browser processes client site code (HTML, JS) -> Browser renders it

So none of the JS doesnt start processing till after the PHP is done. The only way around this to to send a request back to the server via javascript. But I dont know any of that stuff so I cant help you