以下のメソッドで戻り値が取得できないのはなぜでしょうか？

public static function query($sql, $unbuffered = false) {    $ret = self::$db->query($sql, $unbuffered);//该句成功时$ret为1  注：：self::$db->query为mysql_query    if ($unbuffered===true) {      $cmd = trim(strtoupper(substr($sql, 0, strpos($sql, ' '))));      if ($cmd === 'SELECT') { }       elseif ($cmd === 'UPDATE' || $cmd === 'DELETE') {          $ret = self::$db->affected_rows();      }       elseif ($cmd === 'INSERT') {          $ret = self::$db->insert_id();//该处运行时证明执行了，返回的是int类型id号      }   }   return $ret;//最终返回不是int类型id号}       

self::$db->query($sql, $unbuffered);方法如下    public function query($sql, $unbuffered = false) {            $func = $unbuffered ? 'mysql_unbuffered_query' : 'mysql_query';            $query = $func($sql, $this->curlink);            $this->querynum++;            return $query;    }  self::$db->insert_id();方法如下function insert_id() {return ($id = mysql_insert_id($this->curlink)) >= 0 ? $id : @mysql_result($this->query("SELECT last_insert_id()"), false);  }