If you are getting the error:
Your PHP installation appears to be missing the MySQL extension
or
Your PHP installation appears to be missing the MySQL extension which is required by WordPress
while installing WordPress or some other API on your Ubuntu (Linux) machine, then there might be possibility that you have not installed the php-mysql package.
As now, PHP5 is the latest version, so you have to follow the steps below:
simply goto the Terminal (gnome-terminal) through:
Applications -> Accessories -> Terminal
and write the following command:
sudo apt-get install php5-mysql
and restart your apache server by the following command:
sudo /etc/init.d/apache2 restart
If you want to install LAMP on your system using Ubuntu, then the following post can help you:
http://www.webtechquery.com/index.php/2010/01/install-php-mysql-and-apache-lamp-on-ubuntu-linux/