usort with static comparison function

class Thread{

    public static function __compare($thread_a, $thread_b){
        return $thread_a->posts_numbers > $thread_b->posts_numbers ? 1 : ($thread_a->posts_numbers == $thread_b->posts_numbers ? 0 : -1);
    }

}

usort($this->sorted_threads, array('Thread', '__compare'));

 

http://php.net/manual/en/function.usort.php