Laravel: Can't set object values - php

Question

I have a form in Laravel that when it submits, the function donorUpdate is going to be called, and it will save the data to the database. Here is the exact code of the function.
My aim is, the form contains the field name DonorPrefix that when it's empty, the default value should be Mx.. I have tried the code below, and it doesn't work.
public function donorUpdate(Request $request, Donor $donor){
if($request->has('DonorPrefix') == false || $request->DonorPrefix == '' || $request->DonorPrefix == null){
$request->DonorPrefix = 'Mx.';
}
$donor->update($request->all());
return redirect('/donor/'.$donor->DonorID);
}
This is where the weird part start: My debugging attempt is that I commented the return redirect('/donor/'.$donor->DonorID); line and replace with return $request->all(), the value for DonorPrefix is "" (Empty string).
However when I tried return $request->DonorPrefix, the prefix Mx. returned.
Why isn't the value of DonorPrefix is Mx. on $request->all(), but is Mx. on return $request->DonorPrefix?
Edit: I'm certain that other parts of code works like charm. Things are smooth untill I tried adding this 'default value' function.

Answer 1

You can't set request parameters like $request->DonorPrefix = '...'.
To set a parameter on the request you need to use offsetSet like this:
$request->offsetSet('DonorPrefix', 'Mx.');
Also, the has() method already ensures that it is a non-empty string so you can remove some of your checks, and simplify your code like this:
if(!$request->has('DonorPrefix')){
$request->offsetSet('DonorPrefix', 'Mx.');
}