Query including "SET @xxx; @xxx:= @xxx+1" got wrong result set

[PeiJie Lin]

Have someone meet this? 

RedBeanPHP give different result set from native PDO result set when using SQL this, (to get top 5)

     SELECT CASE WHEN @cn = a.cn THEN @rank := @rank + 1 ELSE @rank := 1 END  AS v1,

                   @cn := a.cn                                                                                                AS v2,

                   CASE WHEN @rank <= 4 THEN cust_name ELSE "other" END                     AS v3,

                   SUM(ct)                                                                                                      AS v4

         FROM xxx a

 CROSS JOIN (SELECT @cn := NULL, @rank := 0 ) b 

     GROUP BY v2, v3

     ORDER BY v2, v1 ',

RedBean result by getAll has rows whose rank(v1) > 5, but native PDO returns rows with only rand <=5