gistfile1.txt

$customer = Customers::find()
    ->where(['Customer_id' => $customer_id])
    ->with('Orders')
    ->asArray()
    ->all();

[
    {
        "Customer_id": "3",
        "Serial_number": "102",
        "Name": "Johann Braun",
        "Date_install": "2017-10-04 14:51:53",
        "orders": [
            {
                "Order_id": "5",
                "Customer_id": "3",
                "Order": "228.551",
                "Date": "2017-10-04 15:26:27"
            },
            {
                "Order_id": "11",
                "Customer_id": "3",
                "Order": "120.452",
                "Date": "2017-10-04 15:26:27"
            }
        ]
    }
]