webapp/app/Http/Controllers/LinksController.php

<?php

namespace App\Http\Controllers;

use App\User;
use Illuminate\Http\Request;
use Illuminate\Validation\ValidationException;
use Laravel\Lumen\Routing\Controller as BaseController;

class LinksController extends BaseController
{
    public function add(Request $request)
    {
        /** @var User $user */
        $user = $request->user()->first();
        $email = $user->email;

        $link = $request->input('u');
        $link = filter_var($link, FILTER_VALIDATE_URL);
        if ($link === false) {
            return response()->json(['error' => 'Invalid link.'], 422);
        }

        $title = $request->input('t') ?: '';

        $email_sent = mail($email, 'New link : '.$title, $link);

        if ($email_sent) {
            return response()->json(['info' => 'Email sent.']);
        }

        return response()->json(['error' => 'An error occured while sending email'], 500);
    }
}