call_user_func_array() error on front end

Question

I've read the other posts about this issue, but mine is different in one fundamental way... there is no function defined in the error. It reads like this:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function '' not found or invalid function name in /home/ovbar/public_html/wp-includes/plugin.php on line 496

I've never seen this error without a function defined so I'm at a loss. I moved it using the All-in-One WP Migration plugin. Any ideas? Here's what I've done so far:

• I've already checked for hard-coded urls just to be sure, and there aren't any.
• Deactivating all plugins still shows this error.

Every other aspect of the site works fine. This error only displays on the front end.

Any ideas?

Answer 1

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'));