Please Help - getting a hightlight depending on selected page

Hi can anyone help me please!? I have a page that has a navigation table on the left with various links: Information Software Cars Dishwashers Paper Contact etc... (not the real links ;)) and I want the cell of the section I am in to be highlighted (ie if I am in "Cars" I want the "Cars" link to have a red background). I have set it up so that in CSS the id "sel" is a red b/g. How do I set the id of a certain cell to "sel" taking it from the url (index.php?menu=cars as a very bad example of php coding :)). Cheers Martin