This is something I've needed to do occasionally and always have to look up. The one I like best is located at this post but I'm going to repeat it here for posterity and so I can find it again easier.
perl -pi -w -e 's/search/replace/g;' *.php
So easy, thats why Perl rocks. Thank you Bob Fulkerson for introducing us.
Other options include the frickin sweet looking ruby app SearchAndReplaceGlobal by the same guy who wrote the editor I first learned to do HTML with, Arachnophilia. I say sweet looking because I never actually got it running. I didn't try very hard though.