产生的结果集中size怎样访问

$sql="select user_name,REPLACE(SUBSTRING(SUBSTRING_INDEX(mobile,',',a.id),CHAR_LENGTH(SUBSTRING_INDEX(mobile,',',a.id-1)) + 1),',','') as mobile from table4 a cross join (select user_name,CONCAT(mobile,',') as mobile,length(mobile) - length(REPLACE(mobile,',','')) + 1 size from table1 b) b on a.id<=b.size";

while ($row=mysql_fetch_array($rs)){
echo $row['user_name']."     ".$row['mobile']."    ".$row['size']."<br/>";
}

这样写size无法读出，错误信息是Undefined index:，求大神解答