What browser are you testing on? Netscape 2.0?
Perhaps it's my approach, but I've never had this problem. I've done some very complex stuff with php in iframes as well. I've heard people say this before but I've never been able to reproduce the error on any browser (maybe on IE 4 I did).
Accessibility is great...if you need it to work on everything ever created. Otherwise, excluding a few old browsers is probably a better move. (Hopefully you aren't working on a governmental site).
Are you getting error messages of any sosrt?
'scipt language = php' won't do anything...will not work.