With PHP action you can run any PHP code along the other Elementor Pro's form actions. You can access the form field values through the array $fields, the key of the array are the IDs of the fields.
$fullname = $fields['firstname'] . ' ' . $fields['lastname'];
// ... do something with the full name ...
echo cannot be used in PHP actions. This is because the actions are executed inside an AJAX call. If you want to debug a value we recommend using the plugin Query Monitor with their qm/debug hook.