Upload a file called
phpinfo.php to your site that looks like this:
Then open that page
http://yourcomain.com/phpinfo.php in your browser and share the contents or look for any mentions of the mysql module. This allows you to confirm your site does have that module installed.
Alternative you can use the binary to get that info from the commandline:
php might need to be replaced with the version in use and based on your control panel or hosting environment. Understand that, although this rarely happens, sometimes modules installed for the web aren't installed for the binary because these use different ini-files. So in case the binary says pdo_mysql installed, it might still not be the case for the web php handler.