WordPress Boilerplate Plugin doesn’t see callback functions for add_settings_field and add_settings_section

I needed something like array($this, 'field_callback') instead of just the string 'field_callback' inside the add_settings_xxx functions. I am still seeking an explanation for this solution if you can enlighten me. Why does this work?

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)