PHP5 on Xubuntu installed via Synaptic Package Manager does not come with inbuilt MySQL support. Makes zilcho sense, but it does not. You have to specifically select the PHPMySQL package and install it, it's easy but will save you half an hour of brain racking.

One way to check if your PHP install supoorts MySQL is accessing this function in a PHP file.

Will dump out everything related to your PHP install. There you can do a CTRL-F (Mozilla/IE) for MySQL.