在微信mysql中想实现：当记录不存在时insert,当记录存在时update 出了点问题》》

if($keyword==1||$keyword==2){
            $sql_insert="INSERT INTO `760827`.`session` (`id`,`openid`,`status`) VALUES ('','$fromUsername','$keyword');";
            $sql_update="UPDATE  `760827`.`session` SET  `status` =  '$keyword' WHERE  `session`.`openid` =$fromUsername;";
            $sql_yanzheng="SELECT * FROM `760827`.`session` WHERE 'openid'=$fromUsername;";
            $result_yanzheng=mysql_query($sql_yanzheng,$conn);
            if($keyword==1){
                if(mysql_num_rows($result_yanzheng) > 0){
                    $reply="update";
                    echo _response_text($reply);  

                    //mysql_query($sql_update,$conn);

                }
                elseif(mysql_num_rows($result_yanzheng) == 0){
                    $reply="insert";                //和下面的echo构成微信输出函数
                    echo _response_text($reply);    
                    //mysql_query($sql_insert,$conn);

                }                   
            }
            elseif($keyword==2){
                if(mysql_num_rows($result_yanzheng) > 0){
                    mysql_query($sql_update,$conn);
                }
                else{
                    mysql_query($sql_insert,$conn);
                }
            $reply="请输入您的留言：";
            echo _response_text($reply);
            }
        }