<?php
    if(empty($_POST)){
        ?>
<form action="" method="POST">
    <input name = 'login'></br>
    <input name = 'pass' type="password"></br>
    <input type = "submit">
</form>

<?php
    } else{
    $host = "localhost";
    $user = "root";
    $password = "root";
    $name = "auto";
    $link = mysqli_connect($host,$user,$password,$name);

    if(!empty($_POST['login']) && !empty($_POST['pass'])){
        $login = $_POST['login'];
        $pass = $_POST['pass'];
        $query = mysqli_query($link,"SELECT * FROM users WHERE login='$login' AND password='$pass'");
        $user = mysqli_fetch_assoc($query);
        if(!empty($user)){
            header('Location: index.php');
            echo "Авторизация прошла успешно";
        } else {
            echo "Ошибка авторизации";
        }
    }};
?>