index.php

<?php
include_once("connect.php");
session_start();

if(!empty($_SESSION["id"]) || !empty($_SESSION["username"]))
        header("Location: main.php");

if(isset($_POST["login"])){
    $username = mysqli_real_escape_string($con, $_POST["username"]);
    $password = mysqli_real_escape_string($con, md5($_POST["password"]));
    if(strlen($username) >= 1 && strlen($password) >= 1){
                $result = mysqli_query($con, "SELECT * FROM `UserAccounts` WHERE `username` = '$username' AND `password` = '$password'");
                if($row = mysqli_fetch_array($result)){
                    $_SESSION["id"] = $row["id"];
                    $_SESSION["username"] = $username;
                    header("Location: main.php");
                }
        }
}
?>

<!DOCTYPE html>
<html>
<head>
    <title>Admin Panel</title>
    <link rel="stylesheet" type="text/css" href="styles.css">
</head>
<body>
    <div id="Panel">
        <form method="post" action="<?php $_SERVER["PHP_SELF"];?>">
            <input type="text" name="username" id="Input">
            <input type="password" name="password" id="Input">
            <input type="submit" name="login" id="Button" value="Login">
        </form>
    </div>
</body>
</html>