The error message is quite clear: You have to install the MySQL extension.
See
PHP: Installation - Manual[
^].
But note the comment on top of that page that the MySQL extension is deprecated. If possible you should use one of the new extensions.
If MySQL is installed and you still got the error, check the
extension_dir
setting your
php.ini file. That should be commented (use default directory) or contain a valid path to your PHP extensions (depends on the operating system of your server).