Well, knowing the exact error message usually helps.
However, I have a suspicion that you may need the PHP MySQL module.
If you’re using CentOS, you may need to install the “php5-mysql” package, and you can install it by running “yum install php5-mysql”. Or if you’re using Debian or Ubuntu, it would be called “php-mysql”, and you can install it with “apt-get install php-mysql”.