$obj_car = new class_car;
$num_carId = $obj_car->fn_carAdd($_POST);

class class_car {
    function fn_carAdd($arr_post) {
        print_r($arr_post);
    }
}

Array
(
    [action_post] => carAdd
    [str_postChk] => 
    [str_carName] => 高尔夫
    [num_carBrand] => 7
    [num_carModel] => 615
    [str_carEmiss] => 1.4T
    [str_carTrans] => MT
)

class class_car {
    function fn_carAdd($arr_post) {
        print_r($arr_post["str_carName"]);
    }
}