If you wish to see what PHP extensions are enabled on your version of PHP and what other extensions are available to you, under the SOFTWARE section of your cPanel click on the "Select PHP Version" icon.
From this screen you can change the PHP version for your account and can also control what extensions are enabled.
This will display the native (or default version of PHP) if you have not changed it previously. Please note that you cannot change the default Extensions or Options for the native version of PHP. If you wish to modify these, you would apply that particular version of PHP to your account using the dropdown.
To select a version of PHP to apply, click on the dropdown arrow and a complete list of the available versions of PHP will be shown.
From here you can now see What PHP Extensions are enabled in my version of PHP? and change them for each and every version of PHP you wish to use within your hosting account. Similarly, you can also change the PHP Options used for each version of PHP.
PHP Options are more commonly know as your own "custom php.ini settings". As described in How do I change my php.ini settings or PHP Options?