Unfortunately installing php5 is not that easy on Bubba. This due to dependencies on some already installed packages. Mainly the Bubba package and the webmail application.
That said, it isn't undoable only a bit tricky. (This tip must however be considered a bit advanced.) What you could do to not break other packages is to install php5-cgi that will run php as a cgi-script.
You can find php5 at backports, http://backports.org/
there should be instructions on how to use it.
What you want to do is to install php5-cgi
and then add a handler in the apache configuration to execute files with the suffix .php5 with this installation.
An improvement to this should of course be to use php5 as a fastcgi instead.
A guide to do it the other way around
can be found here: http://www.howtoforge.com/apache2_with_php5_and_php4
this however can't be used on Bubba since we depend on having php4 as a module.
I hope that this at least can be a starter for getting php5 up and running on Bubba.