In the php.ini file, check if the extention path configuration is valid.
You will need to enable the extension=php_mysql.dll option in the php.ini as well. Also, make sure that the file is in the extension_dir you set.
You can read more about it at:
As the others say these two values in php.ini are crucial.
I have the following in my php.ini: note the trailing slash - not sure if it is needed - but it does work.
extension_dir = "H:\apps\php\ext\"
extension=php_mysql.dll
Also it is worth ensuring that you only have one copy of php.ini on your machine - I've had problems with this where I've been editting a php.ini file which php isn't using and getting very frustrated until I realised.
Also if php is running as a module within apache you will need to restart the apache server to pickup the changes. Wise to do this in anycase if you're not sure.
a "php -m" from the cmd prompt will show you the modules that are loaded from the ini file.
On a completely different note, might I suggest WampServer? It should get you up and running with a Apache/PHP/MySQL install in no time.
You could even compare the WampServer config files with your own to see where you originally went wrong.