This instruction is for LAMP technology. For WAMP, I will post later on.

When you have mysql installed in your system, but you do not have mysql.so loaded in your php.ini file. Search for mysql.so in your php extensions directory. If you find it, then simply add mysql.so reference in php.ini. But if you can not find it then remove mysql installation from your system.
After you have removed MySQL completely, look for the php-mysql-5.1.6.rpm in your rpm repository. What this rpm will do is essentially install proper mysql shared object and put the reference to mysql.so in your php.ini.

Make sure you restart Apache server, otherwise the change will not take into effect.