Open the file and change the language in the menu. TM should remember this new association, and treat .php as html or xhtml or whatever you select in the future. In the main menu, I think, there is a 'File associations' item or similar, displaying currently memoized extensions.
it works well.
It is very useful alternate beetween php and xhtml syntax, ok!
Also option+control+shift+p and option+control+shift+h are good things: I must read better the help files *soon* :-))
TextMate is wonderful.