Redirect not working

I’m trying to redirect to another page after I hit the submit button on the form. But it doesn’t work no matter what redirection method I use. It goes to another page giving an error saying that the page doesn’t exist. it doesn’t even give the correct destination of my redirection page. I am only a beginner at php/wordpress so any help would be very much appreciated! because I’m at a loss as to what to do having spent days trying to get it to work.

Here is my code. Please help!

<form action="demo.php" method="POST">
<p>Name<input name="name" type="text" /></p>
<p>Email<input name="email" type="text" /></p>
<p>UserName<input name="user" type="text" /></p>
<p>Password<input name="pass" type="password" /></p>
<p>Confirm Password<input name="cpass" type="password" /></p>
<p><input id="button" name="submit" type="submit" value="Sign Up" /></p>
</form>

<?php

define('DB_NAME', 'wordpress');
define('DB_USER', 'root');
define('DB_PASSWORD', '');
define('DB_HOST', 'localhost');

$link = mysql_connect(DB_HOST, DB_USER, DB_PASSWORD);

if (!$link)
{
	die('Could not connect: ' . mysql_error());

}
$db_selected = mysql_select_db(DB_NAME, $link);

if (!$db_selected){
	die('Can\'t use ' . DB_NAME . ': ' . mysql_error());
}

$fullname = $_POST['fullname'];
$userName = $_POST['userName'];
$email = $_POST['email'];
$password = $_POST['password'];

$sql = "INSERT INTO websiteusers (fullname, userName, email, password) VALUES ('$fullname','$userName','$email','$password')";

if (!mysql_query($sql)) {
	die('Error: ' . mysql_error());
}
if ($data)

    header("Location:http://localhost/wordpress/registration");

mysql_close();

function SignUp()
{
if(!empty($_POST['user']))
{
	$query = mysql_query("SELECT * FROM websiteusers WHERE userName = '$_POST[user]' AND pass = '$_POST[pass]'") or die(mysql_error()); 

	if(!$row = mysql_fetch_array($query) or die(mysql_error()))
	{
		newuser();
	}
 else
	{
		echo "Sorry.  You are already a registered user";
	} 

 }
 } if(isset($_POST['submit']))
 {
 SignUp();
 }