<?php
    session_start(); //Starts A Session Unless There Already Is One.
    include 'dbconnectfilehere.php'; // Connects to the file which accesses the database.

    //Finds if there is a session called 'username', which starts once there logged in!
    if($_SESSION['username']) {
        // If there is a session, it takes them to home.php.
        header('Location: home.php');
    } else {
        //Else if there isn't a session, it see's if there $_POST values exist.
        if(isset($_POST['username']) && isset($_POST['password'])) {
            //If they do, it gets the username and password value.
            $username = mysql_real_escape_string($_POST['username']);
            $password = mysql_real_escape_string(md5($_POST['password']));
            //Now it does a mysql query to try and find the users.
            $query = mysql_query("SELECT * FROM `users` WHERE `username` = '{$username}' && `password` = '{$password}'");
            $num = mysql_num_rows($query);
            if($num == 0) { //If there isn't any rows, it means there information is wrong.
                echo "Wrong information!";
            } else { //If there is a row, it starts the session!
                $_SESSION['username'] = $username;
                $_SESSION['password'] = $password;
                echo "You are now logged in! Please click <a href='home.php'>here!</a>";
            }
        }
        ?>
        <form action="login.php" method="post">
        Username:<br />
        <input type="text" name="username" id="username" /><br />
        Password:<br />
        <input type="password" name="password" id="password" /><br />
        <input type="submit" value="Login" />
        </form>
        <?php
    }
?>