如何将分解的数组（Group_Concat结果）推到另一个数组

Question

要以json_encode格式返回数组，对于单行，只需按如下所示推送元素。例如

$query = "SELECT expenses.ttl AS name, expenses.amnt AS amnt FROM expenses";
$result = $con->prepare($query) or die($con->error);
$result->execute();
$data = array();
foreach($result as $row){
    $data[] = $row;
}
print json_encode($data)

但是如何将GROUP_CONCAT分解数组推入此（下一个）数组。例如

$query = "SELECT GROUP_CONCAT(salary.ttl,',',purchase.ttl,',',extra.ttl) AS name, 
        CONCAT(SUM(salary.amnt),',',SUM(purchase.amnt),',',SUM(COALESCE(extra.amnt,'0'))) AS amnt
        FROM salary LEFT JOIN..."
$result = $con->prepare($query) or die($con->error);
$result->execute();
while($row=$result->fetch(PDO::FETCH_ASSOC)){
    $name = explode(',',$row['name']);
    $amnt = explode(',',$row['amnt']);

    //how to push these specific value to array $data As above.
}

print json_encode($data)