References:
http://www.emacswiki.org/emacs/MuMaMo
http://www.emacswiki.org/emacs/NxhtmlMode
http://ourcomments.org/Emacs/nXhtml/doc/nxhtml.html#php
We download the latest version of Nxhtml from the site: http://ourcomments.org/cgi-bin/emacsw32-dl-latest.pl and specifically the .zip file in the link that says «Download latest nXhtml (zip file)» is enough for us.
We uncompress it wherever we want and add to our .emacs the path to have this mode available.
(load "YOUR-PATH-TO/nxhtml/autostart.el")
We reload our .emacs with M-x load-file .emacs. (This only needs to be done once, and only if you already had emacs open)
We open any .php file and we can already edit with that mode.
M-x nxhtml-mumamo
nxhtml-mumamo uses different colours for the different pieces of code, besides other features.
To learn more, have a look at the tutorials