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<?php
    session_start();
    if (is_logged()) {
        header("Location: welcome.php");
    }
    function login($mysqli,$username,$password){
        $sql = "SELECT * from GEBRUIKERS WHERE username LIKE '{$username}' AND password LIKE '{$password}' LIMIT 1";
        $result = $mysqli->query($sql);
        if (!$result->num_rows == 1) {
            return false;
        } else {
            $_SESSION["username"] = $username;
            return true;
        }
    }
    function is_logged() {
        if (isset($_SESSION['username'])) return true;
        return false;
    }
?>
<html>
<head>
    <title>Login</title>
</head>
<body>
<?php
if (!isset($_POST['submit'])){
?>
    <form action="<?=$_SERVER['PHP_SELF']?>" method="post">
        Username: <input type="text" name="username" /><br />
        Password: <input type="password" name="password" /><br />
        <input type="submit" name="submit" value="Login" />
    </form>
<?php
} else {
    $username = $_POST["username"];
    $password = $_POST["password"];
    require_once("db_const.php");
    $mysqli = new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME);
    if ($mysqli->connect_errno){
        echo "<p>MySQL error no {$mysqli->connect_errno} : {$mysqli->connect_error}</p>";
        exit();
    }
    if(login($mysqli,$usename,$password)){
        header("Location: welcome.php");
    }else{
        echo "Invalid username or password";
    }
    mysqli_close($mysqli);
}

?>
</body>
</html>