$data = array(
   "invoice_info" => array(
       ...
    ),
    "product_info => array(
        array(
           "name" => "product 1",
           ...
        ),
        array(
           "name => "product 2",
            ...
        )
    )
);

$product_info[] = array("name" => "product 1", ...);

[product_info] => Array
    (
        [0] => Array
            (
                [name] => product 2
                [qty] => 293
                [price] => 44
                [sum] => 12892
            )

    )