How to know if there is a record in the database using codeignaiter 3

Depending on how advanced of a function you need you can use the built in function is_unique in form validation.
https://www.codeigniter.com/userguide3/l...ding-rules

If you require more checks you need to write your own callback function and look for it yourself in the database:
https://www.codeigniter.com/userguide3/l...-as-a-rule