<?php
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

class Login
{
    private $login;
    private $password;

    public function __construct($login, $password)
    {
        $this->login = $login;
        $this->password = $password;
    }

    public function login()
    {
        if (empty($_POST['username']) || empty($_POST['password'])) {
            echo 'wypelnij dane';
        } else {
            $connection = mysqli_connect("localhost", "root", "" , "football");
            $result = mysqli_query($connection, "SELECT * FROM user WHERE username = '{$this->login}' and password = '{$this->password}' limit 1");
            if (!$result) {
                die(" Error: " . mysqli_error($connection));
            }
            $check = mysqli_fetch_array($result);
            var_dump($check);
        }
    }
}

$user = new Login($_POST['username'], $_POST['password']);
$user->login();