Help required with a php script

sir On the whole site I could not come across the solution om my probelm. My problem is I have password protected page. The php page verifies the user present in the database and shows him the page. This php page does not check whether the user is already logged on or not to the page on any other computer I do not want that the same username logging on to the page from more that one computer I want that somehow my php programming is such that it verifies whether is user is already logged on or not, if he is logged on then an error message should be shown instead of the page. Please help me with providing with the php script or a link. Thanks Regards neera