I need to find the screen resolution of a users screen who visits my website?
PHP is a server side language - it's executed on the server only, and the resultant program output is sent to the client. As such, there's no "client screen" information available.
That said, you can have the client tell you what their screen resolution is via JavaScript. Write a small scriptlet to send you screen.width and screen.height - possibly via AJAX, or more likely with an initial "jump page" that finds it, then redirects to http://example.net/index.php?size=AxB
Though speaking as a user, I'd much prefer you to design a site to fluidly handle any screen resolution. I browse in different sized windows, mostly not maximized.