I'm not a big fan of Macs, so if you are one, please don't downvote me.
I'm also interested in any comments in general about coding on a Mac and if it's really worth the extra $$$ to get one.
I'd say that, for PHP coding (and any other type of coding that's not Mac-specific), a PC + Linux is a much better and cheaper alternative. I've been using this combo for quite a while now and things have been smooth. The only thing you have to do is look for a Linux-friendly laptop/PC (use sites such as http://www.linlap.com/) and then you're all set. Install Ubuntu (or any distro you prefer) and then you have a myriad of code editors to choose from.
You've got the hardcore vi(m) and emacs, the lightweight but not-featureful gedit, the lightweight but pretty feature packed geany and other, bigger editors such as Aptana Studio, Eclipse, Anjuta DevStudio, Bluefish to name a few. Just browse through these and pick your favorite, and install it using your distro's package manager (in Ubuntu, just go to Applications -> Ubuntu Software Center and you'll find all these in there).
Most of these will support FTP file editing thanks to GVFS (Gnome Virtual File System) which basically allows you to browse, view and edit remote files as if they were on your own PC, and a lot of protocols are supported (FTP and SFTP/SSH included).
As a personal note, I'm using Geany and I have to say it does the job pretty well for me.