change value of $request before validation in laravel 5.5

Question

I have a form in route('users.create').

I send form data to this function in its contoller:

public function store(UserRequest $request)
{

    return redirect(route('users.create'));
}

for validation I create a class in

App\Http\Requests\Panel\Users\UserRequest;

class UserRequest extends FormRequest
{
    public function rules()
    {
        if($this->method() == 'POST') {
             return [
                 'first_name' => 'required|max:250',

It works.

But How can I change first_name value before validation (and before save in DB)?

(Also with failed validation, I want to see new data in old('first_name')

Update

I try this:

 public function rules()
    {
     $input = $this->all();


     $input['first_name'] = 'Mr '.$request->first_name;
     $this->replace($input);

     if($this->method() == 'POST') {

It works before if($this->method() == 'POST') { But It has not effect for validation or for old() function

Answer 1

Override the prepareForValidation() method of the FormRequest.

So in App\Http\Requests\Panel\Users\UserRequest:

protected function prepareForValidation()
{
    if ($this->has('first_name'))
        $this->merge(['first_name'=>'Mr '.$this->first_name]);
}