修改request（）数组返回错误间接修改重载属性

I have a form which includes input names with arrays, examples:

user_profile[birth_day]
user_profile[birth_month]
user_profile[birth_year]
user_profile[birthdate]

now i want to combine them and make a new request property.

i found a solution but i know it's bad practice:

$user_profile['birthdate'] = Carbon::parse(implode("-", [$request->user_profile['birth_day'], $request->user_profile['birth_month'], $request->user_profile['birth_year']]))->format('Y-m-d');
$request->request->add(['user_profile' => $user_profile+$request->user_profile]);

When i get $request->user_profile['birthdate'] it returns the correct input, if filled in. But when i try to modify $request->user_profile['birthdate'] when it is empty with:

$reuest->user_profile['birthdate'] = Carbon::parse(implode("-", [$request->user_profile['birth_day'], $request->user_profile['birth_month'], 
$request->user_profile['birth_year']]))->format('Y-m-d');

i get the following error:

Indirect modification of overloaded property Illuminate\Http\Request::$user_profile has no effect

You can try this, I guess this will work

$reuest->user_profile->birthdate = Carbon::parse(implode("-", [$request->user_profile['birth_day'], $request->user_profile['birth_month'], $request->user_profile['birth_year']]))->format('Y-m-d');