Yes, and yes.
You can use Gusto or similar on your iPad to edit php files on a server (sort of, that is, you download them, edit them and upload them again). Just fine for making a small tweak on the go, IMO, but for real development you're better off with a small laptop, full keyboard (easier typing and access to special characters), your text editor of choice (mine's Coda) and the possibility to run MAMP or similar for local/offline development, too.