How to set private class to form validation callback in codeigniter
You can just declare your private function as you have already done:
function _username_check(){ // some code}And in the validation rule, use:
callback__username_checkAs I have seen, this must work just fine!
Remember:
The _ prefix would take care of your function privacy, so you do not really have to use the keyword private to declare the function to let the form_validation class access to that function!