php-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