Register.php

<?php
session_start();
include realpath("./inc/class.template.php");

$template->_get("header.html");
$template->_set(array(
    'TZ_TITLE' => 'Tezzy',
    'BB_SUBT' => 'Index',
    'BB_USER' => 'Username'
 ));

if(isset($_POST['submitR'])) {

$username = $_POST['username'];
$password = $_POST['password'];
$passwordC = $_POST['passwordC'];
$email = $_POST['email'];
$ipaddress = $_SERVER['REMOTE_ADDR'];
$session = session_id();

if($username == '' or $password == '' or $passwordC == '' or $email == '') {
  header("Location: ./register.php?error=1"); // Tells to fill in all fields
} elseif ($password != $passwordC) {
  header("Location: ./register.php?error=2"); // Checks to see if password is correct
} elseif (mysql_num_rows(mysql_query("SELECT * from users WHERE username='" . $_POST['username'] . "'")) == 1) {
  header("Location: ./register.php?error=3"); // Checks to see if username is taken
} elseif (mysql_num_rows(mysql_query("SELECT * from users WHERE email='" . $_POST['email'] . "'")) == 1) {
  header("Location: ./register.php?error=4"); // Checks to see if email is taken
} else {
  function sha512($str) { return hash("sha512", $str); }
  $password = sha512($password);
  mysql_query("INSERT INTO users (username,password,email,ip,session) VALUES('{$username}','{$password}','{$email}',{$ipaddress}','{$session}')");
  header("Location: ./register.php?success=1");
 }
}

$template->_get("register_body.html");
$template->_set(array(
    'TZ_TITLE' => 'Tezzy'
 ));

$template->_get("footer.html");
$template->_set(array(
    'BB_TITLE' => 'Tezzy'
 ));
?>

Register Template:

<form action="./register.php" method="POST">
Username: <input type="text" name="username"><br />
Password: <input type="password" name="password"><br />
Password Confirm: <input type="password" name="password"><br />
Email: <input type="password" name="password"><br />
<input type="submit" name="submitR" value="Register">
</div>