User Registration and Login System Using PHP and MySQL

<?php
/*****************************
	File: register.php
	Written by: Frost of Slunked.com
	Tutorial: User Registration and Login System
******************************/
require($_SERVER['DOCUMENT_ROOT'] . 'login/includes/config.php');
 
$sOutput .= '<div id="register-body">';
 
if (isset($_GET['action'])) {
	switch (strtolower($_GET['action'])) {
		case 'register':
			// If the form was submitted lets try to create the account.
			if (isset($_POST['username']) && isset($_POST['password'])) {
				if (createAccount($_POST['username'], $_POST['password'])) {
					$sOutput .= '<h1>Account Created</h1><br />Your account has been created. 
								You can now login <a href="login.php">here</a>.';
				}else {
					// unset the action to display the registration form.
					unset($_GET['action']);
				}				
			}else {
				$_SESSION['error'] = "Username and or Password was not supplied.";
				unset($_GET['action']);
			}
		break;
	}
}
 
// If the user is logged in display them a message.
if (loggedIn()) {
	$sOutput .= '<h2>Already Registered</h2>
				You have already registered and are currently logged in as: ' . $_SESSION['username'] . '.
				<h4>Would you like to <a href="login.php?action=logout">logout</a>?</h4>
				<h4>Would you like to go to <a href="index.php">site index</a>?</h4>';
 
// If the action is not set, we want to display the registration form
}elseif (!isset($_GET['action'])) {
	// incase there was an error 
	// see if we have a previous username
	$sUsername = "";
	if (isset($_POST['username'])) {
		$sUsername = $_POST['username'];
	}
 
	$sError = "";
	if (isset($_SESSION['error'])) {
		$sError = '<span id="error">' . $_SESSION['error'] . '</span><br />';
	}
 
	$sOutput .= '<h2>Register for this site</h2>
		' . $sError . '
		<form name="register" method="post" action="' . $_SERVER['PHP_SELF'] . '?action=register">
			Username: <input type="text" name="username" value="' . $sUsername . '" /><br />
			Password: <input type="password" name="password" value="" /><br /><br />
			<input type="submit" name="submit" value="Register!" />
		</form>
		<br />
		<h4>Would you like to <a href="login.php">login</a>?</h4>';
}
 
$sOutput .= '</div>';
 
// display our output.
echo $sOutput;
?>