求问以次方法为何不能得到返回值

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