Compare two pages with the url name, previous page compare witht tu

i just want to compare the previous and current page name...
is this possible that if i have a form containing some data field like user name, page hase some var , without whoom it can access the login page, so "i just want to comare two pages , if the condition is true the the user can acces that page otherwise he can not, and will be terminated to the filed without any logiin details...
plz help meee"