Your PHP installation appears to be missing the MySQL extension which is required by WordPress
How do I fix this problem under Debian or Ubuntu Linux?
First, make sure MySQL server is running. Type the following command at a shell prompt:
# /etc/init.d/mysql status
If MySQL is not running, enter:
# /etc/init.d/mysql start
If MySQL is not installed, type the following command to install MySQL server:
# apt-get install mysql-server
Make sure MySQL module for php5 is installed:
# dpkg --list | grep php5-mysql
To install php5-mysql module enter:
# apt-get install php5-mysql
Next, restart the Apache2 web server:
# /etc/init.d/apache2 restart
Now PHP support for MySQL should work without a problem. Also make sure you set localhost or 127.0.0.1 as MySQL hostname while performing wordpress installation.