function delete_quiz($quiz_id)
 {
     //delete questions
     $query = he_database::placeholder("SELECT `id` FROM `se_he_quiz_question` \r\n            WHERE `quiz_id`=?", $quiz_id);
     $question_ids = he_database::fetch_column($query);
     he_quiz::delete_questions($question_ids);
     //delete results
     $query = he_database::placeholder("SELECT `id` FROM `se_he_quiz_result`\r\n            WHERE `quiz_id`=?", $quiz_id);
     $result_ids = he_database::fetch_column($query);
     he_quiz::delete_results($result_ids);
     //delete plays
     $query = he_database::placeholder("DELETE FROM `se_he_quiz_play`\r\n            WHERE `quiz_id`=?", $quiz_id);
     he_database::query($query);
     //get quiz photo
     $query = he_database::placeholder("SELECT `photo` FROM `se_he_quiz`\r\n            WHERE `quiz_id`=?", $quiz_id);
     $photo = he_database::fetch_field($query);
     if ($photo) {
         //delete photo
         @unlink(he_quiz::photo_dir() . $photo);
     }
     //delete comments
     $query = he_database::placeholder("DELETE FROM `se_he_quizcomments`\r\n            WHERE `he_quizcomment_quiz_id`=?", $quiz_id);
     //delete quiz
     $query = he_database::placeholder("DELETE FROM `se_he_quiz` WHERE `quiz_id`=?", $quiz_id);
     he_database::query($query);
 }