call_user_func_array() error on front end

I’ve successfully tracked this problem by adding

error_log("wp-hook: ". print_r($the_, true));

within apply_filters in wp-includes/wp_hook.php

The task is then to look at the function calls (the data inside $the_) surrounding the “PHP Warning: call_user_func_array()”. It will be the one directly ahead of the warning, but you may need to look at calls after or before to determine which file is causing the problem (a filter call to an undefined function).

Note also that the problem will arise if the function exists, but has no code, like:

function create_menu() {
    //oops, there's no code here
}
add_action('admin_menu',array($this,'create_menu'));