This error message is displayed when you haven't installed or activated the MySQL library used by PHP. Try to find the mysql.so file by using the following command:

find / -name mysql.so

 When you find this file, activate the extension in your php.ini file (this file is usually located in the directory /etc/php4/apache/) by typing option:

extension=mysql.so

You can use Midnight command (mc) program to edit this file. If you don't find the mysql.so file, try to install a new package php4-mysql (package name php4-mysql is used by Debian, in Fedora distribution it will be found with the same or similar to the Debian's name).