SqlHelper::execute_dql PHP Method

public function execute_dql($sql){

$res=mysql_query($sql,$this->conn) or die(mysql_error());
return $res;

}

 public function ChangePassword($username, $oldPassword, $newPassword)
 {
     $result = false;
     $sql = "select password from t_admin where username='******'";
     //创建一个SqlHelper对象
     $sqlHelper = new SqlHelper();
     $res = $sqlHelper->execute_dql($sql);
     if ($row = mysql_fetch_assoc($res)) {
         //比对密码
         if (md5($oldPassword) == $row['password']) {
             $sql = "update t_admin set password='******' where username='******'";
             if ($sqlHelper->execute_dml($sql) != 0) {
                 $result = true;
             } else {
                 $this->errMessage = "更新失败！";
             }
         } else {
             $this->errMessage = "原始密码不正确";
         }
     }
     //资源
     mysql_free_result($res);
     //关闭链接
     $sqlHelper->close_connect();
     return $result;
 }