<?php
    if(isset($_GET['sign-out'])) {
    unset($_SESSION['sign-in']);

        header("Location: index.php");
        die;
    }
    if(!isset($_SESSION['sign-in'])) {
?>
<form method="POST" action="index.php">
    <label for="username">Username</label>
    <input type="text" id="username" name="username" placeholder="Username" tabindex="1">
    <br><br>
    <label for="password" id="password" name="password">Password&nbsp;</label>
    <input type="password" id="password" name="password" placeholder="Password" tabindex="2"><br><br>

    <input type="submit" name="submit" value="Login" tabindex="3">
</form>
<?php
    if(isset($_POST["submit"])) {
        // require "app/includes/function.php";

        $username = htmlspecialchars($_POST['username'], ENT_QUOTES);
        $password = htmlspecialchars($_POST['password'], ENT_QUOTES);

        $query = mysqli_query($connection, "SELECT `id` FROM `users` WHERE `user_name` = '".$username."' AND `user_password` = '".$password."'"); die;
        $rows = mysqli_num_rows($query);

        if($rows > 0) {
            $_SESSION['sign-in'] = "true";

            header("Location: index.php");

            die;
        } else {
            echo "The data is not correctly entered";
        }
    }
} else {
?>
    You have successfully logged<br>
    <a href="index.php?sign-out=true">Sign out</a>
<?php
    }
?>