<?php
$servername="localhost";
$username ="papryk";
$password = "1234";

$conn = new mysqli($servername, $username, $password, "logowanie");

if($conn->connect_error){
    die("Connection failed: " . $conn->connect_error);
}

$e_mail = $_POST['login'];
$haslo = $_POST['haslo'];

$wynik = $conn->query("SELECT * FROM `users` WHERE `e-mail`='$e_mail'");

    if ($wynik->num_rows > 0) {
        $row = $wynik->fetch_assoc();
        //echo "<br> id: " . $row["id"]. " - marka: " . $row["marka"]. " model: " . $row["model"]. " pojemnosc: " . $row["pojemnosc"]. "<br>";
        if($haslo == $row["haslo"])
            echo "Witaj ". $row["imie"]. " ". $row["nazwisko"]. "<br>Jesteś z nami od ". $row["data_zal"];
    }
    else
        echo "Błędny login lub hasło";

?>

<button class="wyloguj">Wyloguj się</button>

<script>
document.querySelector('.wyloguj').addEventListener("click",function(){
    <?php $conn->query("session_destroy()") ?>
});
</script>