I can\'t seem to disable error reporting in PHP - I have tried everything but \"Notice\" errors are still displayed.
My php.ini has
display_errors = Off;
As @colmde already pointed OpenCart uses custom error_handler.
You can turn error displaying off without any code edits (especially OpenCart core files) via:
Admin->System->Settings->[edit your configured store]->Server->Display Errors
[EDIT] You can do the same by running following query against OpenCart database:
update setting set `value`= 1 where `key` = 'config_error_display'