The <input> generated by your client_id_field() function is using the wrong name which is pardot_client_id, and by “wrong”, I mean there’s no database option registered with that name and the one you’ve registered is named pardot_settings.
-
The database option is the second parameter for
register_setting(). -
And it should be used in the
nameattribute of form fields like<input>and<select>, e.g.<input name="DATABASE OPTION NAME">.
So looking at your class, you should have used <input type="text" size="30" id="pardot_client_id" name="{$html_name}" value="{$client_id}" /> there.
And then get the saved option like so:
// Get all settings in the option array.
$options = (array) get_option( self::$option_group );
// Get the pardot_client_id setting.
$client_id = $options['pardot_client_id'] ?? '';