If you meant "functionality" because parts of the page don't display correctly - welcome to web development.
IE7/8/9, Firefox, Chrome, Opera all have different rendering engines (or versions thereof) so they don't all display the same code the same way.
My approach is generally to make a standards compliant page - run it through the W3C's validators. If your page doesn't validate, each browser could be rendering the page in who knows how many ways. Once your page validates, check again. If it still looks weird, start playing around with CSS & do lots of Google searching until you figure out why.
Also ensure that your DTD is correct, otherwise IE will go to quirksmode - then you have all sorts of rubbish.
BTW, this is a php forum. I would be very surprised if the problem lies with php. It could be your DTD or more likely your CSS. JS can be different, but using a framework like jQuery usually solves these issues.