form.php

<?php

    $username = $_POST["username"];
    $password = $_POST["password"];

    $host="localhost";
    $dbusername="";
    $dbpass="";
    $db_name="prestiz";
    $con = mysqli_connect("$host", "$dbusername", "$dbpass", "$db_name")or die("cannot connect" . mysqli_error($con));

    $result = mysqli_query($con, "SELECT * FROM radko WHERE username = '$username' AND password = '$password'")
        or die("Failed to query database" . mysqli_error($con));


    if(mysqli_num_rows($result)==1){

        echo ("Spravny heslo!");

    }else {

        echo ("Spatny heslo!");
    }


?>