would be "lamp"
linux apache mysql php
which, in all fairness, would also include freebsd, openbsd, postgresql and perl, python, c, c++, delphi.. ( any programming language actually )
with the open source os you can use any executable as cgi, so coding something for use in a web app as cgi is a good skill to have.
certification, use the most respected for your location, usually comptia in the us.
try to avoid a distro centric certification or outlook as they can inhibit using the best tools available.
RHEL, great if you want completely non standard system
Suse, haven't been able to install, my bios doesn't like thier scsi interface requirement.
Debian, one of the distros closest to the intent, pure community driven.
Mandriva, a growing concern, with a decent set of tools.
Slakware, one of the oldest, and closest to linux ideals in being a community distro.
the list is huge, there are well over 40 different distros. almost all have a livecd version.
if you have options as to which will be used, get the livecd for any distro that catches your eye and boot up, see if it does have what you want and need. then either download the full distro ( community distros and versions this is pretty much only method ) or purchase a boxed set of the distro you choose.
to get fully into the core of linux, lfs.
building a linux system from scratch does teach a lot of what is actually required for a distro, but it's not for someone that is going to be using linux as end user. ( document writing..spreadsheet work etc.. )
a system / network admin can make great use of the knowledge from doing so. a developer also.