Php can detect IP, hostname, client agent etc. Can php detect client browser monitor size/resolution?
Note, that JS can check the window size of browser, but this size includes user toolbars, scrollbars etc... Real workspase area in browser depends on those toolbars size.
Some people require browser size for mobile devoloping. This is essential information in some cases.
Using WURFL and WALL can get around this as most mobiles do not support JS.