Add your favourite debian sarge repository to /etc/apt/sources.list, for instance:
deb http://ftp.de.debian.org/debian sarge main contrib
And run the following command prior to installing php5:
$ apt-get update
I should be ok. Every new created virtual host, you have a special directory (/etc/apache2/sites-available/user-ebox-$vhostName) to include every configuration file you need.
I hope this helps.