I've been pulling my hair out trying to get this working. There is a file in core that throws up errors using the SagePay extension from Circle. I can edit the core .php file and fix the problem. Obviously each time I update it is overwritten. So I have created the same directory structure under civicrm_custom_php and added the edited file. Directory paths are correct, but the file in custom isn't being picked up. Any hints or tips really appreciated on getting this working. I am running CiviCRM 5.6.0 on Drupal 7.