index.php

<?php session_start(); ?>
<!DOCTYPE html>
<html lang="en">
<head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" integrity="sha384-/Y6pD6FV/Vv2HJnA6t+vslU6fwYXjCFtcEpHbNJ0lyAFsXTsjBbfaDjzALeQsN6M" crossorigin="anonymous">
</head>
<body>
<div class="container">
    <div class="row">
        <form method="POST">
            <div class="form-group">
                <label for="exampleInputEmail1">Email address</label>
                <input type="email" class="form-control" id="exampleInputEmail1" aria-describedby="emailHelp" placeholder="Enter email" name="email">
                <small id="emailHelp" class="form-text text-muted">We'll never share your email with anyone else.</small>
            </div>
            <div class="form-group">
                <label for="exampleInputPassword1">Password</label>
                <input type="password" class="form-control" id="exampleInputPassword1" placeholder="Password" name="pass">
            </div>
            <div class="form-check">
                <label class="form-check-label">
                    <input type="checkbox" class="form-check-input">
                    Check me out
                </label>
            </div>
            <input type="submit" name="login" class="btn btn-primary" value="Zaloguj" />
            <input type="submit" name="logout" class="btn btn-primary" value="Wyloguj" />
        </form>
    </div>
</div>

<!-- Optional JavaScript -->
<!-- jQuery first, then Popper.js, then Bootstrap JS -->
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.11.0/umd/popper.min.js" integrity="sha384-b/U6ypiBEHpOf/4+1nzFpr53nxSS+GLCkfwBdFNTxtclqqenISfwAzpKaMNFNmj4" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/js/bootstrap.min.js" integrity="sha384-h0AbiXch4ZDo7tp9hKZ4TsHbi047NrKGLO3SEJAg45jXxnGIfYzk4Si90RDIqNm1" crossorigin="anonymous"></script>
</body>
</html>

<?php
require_once "connect_db.php";

$conn = new connect_db();

$email = $_POST['email'];
$pass = $_POST['pass'];

if(isset($_POST['login'])){

    if(isset($_POST['email'])) {
        if ($isUser = $conn->checkUser($email, sha1($pass))) {

            $_SESSION['login'] = $conn->nameUser($email);
            $_SESSION['user_id'] = $conn->idUser($email);
        } else echo "nie ma takiego użytkownika";
    }

}

if(isset($_POST['logout'])){
    unset($_SESSION['user_id']);
    unset($_SESSION['login']);
}

if(isset($_SESSION['user_id'])){
    echo 'Zalogowany jako '.$_SESSION['login'];
}