Intercepting a add_action

Assume that some other plugin or theme uses a add_action hook. Is there a way to determine if a specific hook is being used by someone else?

Example: assume that a plugin has this code

add_action ( 'publish_post', 'myCustomFunction' );
function myCustomFunction() {
   // does something
}

Can my plugin ‘look’ for the use of that hook into ‘publish_post’? And can I remove that particular action hook before it fires?

You can use has_action(). It returns the priority of the hook if it exists, otherwise false. So check that the priority is not equal to false (and not just false-y, or being hooked at 0 will appear false) then use that to remove the hook.

$priority = has_action( 'publish_post', 'myCustomFunction' );

if ( $priority !== false ) {
    remove_action( 'publish_post', 'myCustomFunction', $priority );
}

There’s also has_filter(), which does exactly the same thing (has_action() just calls has_filter()).