form_validation.php in config and conditional validation rules

1st option: Callback documentation https://codeigniter.com/user_guide/libra...on-methods

2nd option: Create config, load that and get item of that https://www.codeigniter.com/userguide3/l...onfig.html
Than, merge array with definition of rule using model's method
http://php.net/manual/en/function.array-merge.php