Form Registration not getting a error - php

Question

so i edited my code but still unable to get the results part of code is responsible for reporting error that do not work too
i'm open to other ways of coding to do the work as long as it fits in the code here comes the code
<?php
require 'core.inc.php';
if(!loggedIn()) {
//check mikunim ke tamame field ha dar form vojod darand va set shudan
if(isset($_POST['username'])&&isset($_POST['password'])&&isset($_POST['password_again'])&&isset($_POST['firstname'])&&isset($_POST['surname'])) {
$username = $_POST['username'];
$password = $_POST['password'];
$password_again = $_POST['password_again'];
$firsname = $POST['firstname'];
$surename = $POST['surename'];
//HALA CHECK MIKUNIM KHALI HASTAND YA NA
if(!empty($username)&&!empty($password)&&!empty($email)&&!empty($firstname)&&!empty($surename)){
echo 'ok' ;
} else {
echo ' All fields are required';
}
}
?>
<form action="register.php" method="POST">
Username:<br> <input type="text" name="username"><br> <br>
Password:<br> <input type="password" name="password"><br><br>
Password again:<br> <input type="password" name="password_again"><br><br>
Firstname:<br> <input type="text" name="firstname"><br><br>
Surname:<br> <input type="text" name="surename"><br><br>
<input type="submit" value="register">
</form>
<?php
} else if (loggedIn()) {
echo 'you \'re already logged in';
}
i edited again now i get this
Notice: Undefined index: HTTP_REFERER
$http_referer = $_SERVER['HTTP_REFERER']
i used this from tutorial

Answer 1

change this
$firsname = $POST['firstname'];
$surename = $POST['surename'];
to this:
$firstname = $_POST['firstname'];
$surename = $_POST['surename'];