NO, TextEdit is NOT a wise tool for editing PHP files. Perhaps for HTML, but not for PHP.

Aptana Studio is a great tool for editing programming code, including PHP. Good choice!
It can also be set up to interact with your website, but I echo tony_sar's comments: I STRONGLY suggest that you ALWAYS do your edits locally and only upload when done testing. There are several good FTP applications for the Mac -- use one that will Synchronize your changed files, and that'll make life super easy!