<?php
$conn = mysqli_connect("localhost", "root", "", "login");

if (isset($_POST['inloggen'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];

    $query = "SELECT * FROM users WHERE username = '" . $username . "' AND '" . $password . "'";

    $result = mysqli_query($conn, $query);

    if (mysqli_num_rows($result) == 1) {
        echo "Juiste gegevens!";
    } else {
        echo "Onjuiste gegevens!";
    }

    echo "<br />";
}
?>

<form method="post" action="">
    <label>Username</label>
    <input type="text" name="username"/><br />
    <label>Password</label>
    <input type="password" name="password"/><br />

    <input type="submit" name="inloggen" value="Inloggen"/>

</form>