#Actual Login Script.
function do_login()
{
        //If the user has submitted the form
        if($_POST['submit']){
            //protect the posted value then store them to variables
            $username = protect($_POST['username']);
            $password = protect($_POST['password']);
            $password = md5($password);

            //Check if the username or password boxes were not filled in
            if(!$username || !$password){
                //if not display an error message
                echo "<center>You need to fill in a <b>Username</b> and a <b>Password</b>!</center>";
            }else{
                //if the were continue checking

                //select all rows from the table where the username matches the one entered by the user
                $res = mysql_query("SELECT * FROM `users` WHERE `username` = '".$username."'");
                $num = mysql_num_rows($res);

                //check if there was not a match
                if($num == 0){
                    //if not display an error message
                    echo "<center>The <b>Username</b> you supplied does not exist!</center>";
                }else{
                    //if there was a match continue checking

                    //select all rows where the username and password match the ones submitted by the user
                    $res = mysql_query("SELECT * FROM `users` WHERE `username` = '".$username."' AND `password` = '".$password."'");
                    $num = mysql_num_rows($res);

                    //check if there was not a match
                    if($num == 0){
                        //if not display error message
                        echo "<center>The <b>Password</b> you supplied does not match the one for that username!</center>";
                    }else{
                        //if there was continue checking

                        //split all fields fom the correct row into an associative array
                        $row = mysql_fetch_assoc($res);


                            //set the login session storing there id - we use this to see if they are logged in or not
                            $session_id = $row['user_id'];
                            $session_username = $username;
                            $session_level = $row['permission'];

                            $_SESSION['user_id'] = $session_id;
                            $_SESSION['permission'] = $session_level;
                            $_SESSION['username'] = $session_username;

                            //show message
                            echo "<center>" . $_SESSION['username'] . ", you have successfully logged in!</center>";
                            header('Location: index.php');
                        }
                    }
                }
            }

}