Error: call_user_func_array() expects parameter 1 to be a valid callback

I have issue after upgrading my wordpress to 3.6 see below for errors which displays on wordpress admin panel not on front of website.

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘regis_options’ not found or invalid function name in wp-includes/plugin.php on line 406

Warning: Cannot modify header information – headers already sent by (output started at wp-includes/plugin.php:406) in wp-includes/option.php on line 571

Warning: Cannot modify header information – headers already sent by (output started at wp-includes/plugin.php:406) in wp-includes/option.php on line 572

Somewhere in your theme or plugins is a line like this:

add_filter( 'something', 'regis_options' );

Could also be add_action(). Find that piece of code and remove or fix it.

The other errors are a result of the first one. The printed error message causes output and hence HTTP headers, so PHP/WP cannot send other headers anymore. They will go away when you fix the first error.