as title says: Is it possible for php to check for file on users computers?
its not possible. PHP is wholly client side and the only time it ever even comes close to interacting with the client side is during a form and that is only communicating with the browser and information the browser sends. so no.
Err… I think you mean PHP’s server side, not client-side. 
And no, it’s not possible. It’d be a huge security concern.
But php can check for the presence of cookies right?
Yes. It’s in the super-global array $_COOKIE…
It is just requesting to see if there are any, your computer doesn’t have to respond to its request.
Indirectly you can of course
