laravel-beginning/form-app/app/Http/Controllers/FormController.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\File;

class FormController extends Controller
{
    public function index() 
    {
        return view('form');
    }

    public function submit(Request $request)
    {
        $validated = $request->validate([
            'name' => 'required|string|min:3|max:15',
            'email' => 'required|email|max:50',
            'message' => 'required|string|min:10|max:100'
        ]);

        $path = storage_path('app/forms/contacts.json');

        File::ensureDirectoryExists(dirname($path));

        $data = [];
        if (File::exists($path)) {
            $content = File::get($path);
            $data = json_decode($content, true) ?: [];
        }

        $validated['created_at'] = now()->toDateTimeString();
        $data[] = $validated;

        File::put($path, json_encode($data, JSON_UNESCAPED_UNICODE));

        return redirect()->back()->with('success', 'Данные сохранены!');
    }
}