There is no reason at all why PHP code would "screw up" in one browser but not in another. It's serverside. Not client side. Perhaps the HTML that the PHP outputs "screws up" IE and Opera, but not the PHP itself.
The menu which appears on the non-Q&A pages (i.e. the index page) (on the left, hover over the 'Webmistress' etc text) doesn't work in IE only on the Q&A pages.
Also, in IE the content box doesn't stretch to 50% and the boxes behind the menu text also go smaller.
I'm not sure how better to explain than look at the Q&A page in IE and in FF...