When you apt-get install php5 on a Debian/Ubuntu server, you’ll notice that APT will automatically install a bunch of apache2 packages as well. This can be pretty annoying if you’re planning on using another web server (or no web server at all).
If you take a look at the package dependencies (Debian/Ubuntu) you’ll see why this happens - php5 needs one of either libapache2-mod-php5, libapache2-mod-php5filter, php5-cgi, or php5-fpm. APT doesn’t care which package it installs; it just picks the first package that satisfies the dependency, which is why you get the apache2 packages.
You can get around this by installing one of the other dependencies before php5. For example, apt-get install php5-fpm php5 or apt-get install php5-cgi php5.