Untitled

<!DOCTYPE html>
<?php
     if(isset($_POST['username']) && isset($_POST['psw'])){
        $username = $_POST['username'];
        $psw = $_POST['psw'];
        $con = mysqli_connect('localhost','root','','cms');
        $sql = "select imie, nazwisko, username, password from users where username='$username' and password='$psw'";
        $wynik = mysqli_query($con,$sql);
        if($wynik){
            while($wiersz = mysqli_fetch_assoc($wynik)){
                if($username == $wiersz['username'] && $psw == $wiersz['password']){
                    $_SESSION['imie'] = $wiersz['imie'];
                    $_SESSION['nazwisko'] = $wiersz['nazwisko'];
                      $_SESSION['logged']=1;
                }
                else{
                    echo "<script>alert ('Błędny login lub hasło');</script>";
                    header("Location:index.php");
                }
            }
        }
        else{
            echo "Błąd połączenia z bazą danych";
            exit;
        }

        }

    if(!isset($_SESSION['logged'])){
    ?>
     <div id="login">
               <button onclick="document.getElementById('id01').style.display='block'">Zaloguj</button>
            </div>
               <div id="id01" class="modal">

                <form class="modal-content animate" action="index.php" method="post">
                    <div class="modal-container">
                        <span onclick="document.getElementById('id01').style.display='none'" class="close" title="Close login modal">&times;</span>
                        <label for="username">
                            <b>Użytownik</b>
                            <input type="text" placeholder="Wpisz nazwę użytkownika" name="username" id="username" required>
                             <br />
                        </label>
                        <label>
                            <b>Hasło</b>
                            <input type="password" placeholder="Wpisz hasło" name="psw" required>
                            <br>
                        </label>
                        <button type="submit">Zaloguj</button>
                    </div>

                    <div class="modal-container" style="background-color:#f1f1f1">
                        <button type="button" onclick="document.getElementById('id01').style.display='none'" class="cancelbtn">Anuluj</button>
                    </div>
                </form>
            </div>
   <?php

    }
    else{
        echo "<a href=\"index.php?logout=1\"><button type='button' id='logout'>Wyloguj!</button></a>";
    }
?>