5
I had the same simptoms in Knoppix, but in Fedora in was easy to start.
So... simptoms in Knoppix:
1. phpMyAdmin is working very good, but XOOPS is crashes while opening connection with mySQL, so it shows blank (checked with "hello world" method).
2. phpinfo() function gives many info I don't need, and also shows that mySQL name exists only as configure-command.
Now I'm pretty new with Debian, so I was off the ground when I found package configure button in Synaptic
So, with its help checked php4-mysql package to work in php.
It's working
And with phpinfo() I can see the whole section for mysql.
So, it's possible that you have bad configuration, or bad, not all packages.