For that task I recommend you to install XAMPP. It is a distribution of PHP, MySQL and Apache configured and ready to use. There is a package in the extra repo: http://vectorlinux.osuosl.org/veclinux-5.8/extra/net/xammp-1.5.5a-i586-4vl58.tlz
For some reason I was unable to get it with slapt-get, I think there is something wrong with the names there, you can give it a try, may be is just my box.
The repo version is a little old, 1.5.5a, the current XAMPP is 1.6.3. If you are interested in geting the last version, you can get it from here.
It is very easy to install.
If you still want to go with the configurations of the slackware packages I recommend you to find some howto on the net and read it carefully. This is not an easy task. Is not impossible, but requires some experience and patience. The tldp project is a good place to start, the stuff there is always a little old, but you can trust in the contents: http://tldp.org/HOWTO/
Another one could be http://lamphowto.com/
Dont pay attention to the rpm part. The section about configuring the server is pretty good.
Please post questions here if you want to take the long way and you find any troubles
. If you take the short one, enjoy XAMPP, I am doing it
Hope this help.