PHP - Variable not echoing - php

Question

<?php
$output = NULL;
$ip = $_SERVER['REMOTE_ADDR'];
if (isset($POST['submit'])) {
$username = $mysqli->real_escape_string($_post['username']);
$password = $mysqli->real_escape_string($_post['password']);
$rpassword = $mysqli->real_escape_string($_post['rpassword']);
$email = $mysqli->real_escape_string($_post['email']);
$query = $mysqli->query("SELECT * FROM users WHERE username = '$username'");
if (empty($username) OR empty($password) OR empty($email) or empty($rpassword)) {
$output = "Please fill in all fields!";
} elseif ($query->num_rows != 0) {
$output = "That username is already taken!";
} elseif ($rpassword != $password) {
$output = "Password does not match!";
}
}
?>
Later on in the script, I use this:
<?php
echo $output;
?>
It does not echo, and yes, I have added the mysqli query, but I removed it for the safety of the database. You can also see that it does not echo at the website:
vobern.com

Answer 1

PHP is a case-sensitive Language. There is a difference between $_POST AND $_post. You may also want to take that into consideration. Now why don't you try doing it like below?
<?php
$output = NULL;
$ip = $_SERVER['REMOTE_ADDR'];
if(isset($_POST['submit'])){
// FOR THE VARIABLES BELOW, THERE IS A DIFFERENCE BETWEEN
// $_POST AND $_post (AS YOU WROTE)....
$username = htmlspecialchars(trim($_POST['username']));
$password = htmlspecialchars(trim($_POST['password']));
$rpassword = htmlspecialchars(trim($_POST['rpassword']));
$email = htmlspecialchars(trim($_POST['email']));
$query = $mysqli->query("SELECT * FROM users WHERE username = '$username'");
if (empty($username) || empty($password) || empty($email) || empty($rpassword)){
$output = "Please fill in all fields!";
}elseif($query->num_rows != 0){
$output = "That username is already taken!";
}elseif ($rpassword != $password){
$output = "Password does not match!";
}
}else{
// IF THIS POINT IS REACHED, THEN EVERYTHING SHOULD BE OK
$output = "Login Data is Correct";
}
var_dump($output);
?>

Answer 2

as pointed by DarkBee in comment you have many errors
if(isset($POST['submit'])) replace this with if(isset($_POST['submit']))

Answer 3

All $_post in your code should be $_POST in upper case.
You add one more else to the last
elseif ($rpassword != $password){
$output = "Password does not match!";
}else{
$output = "Valid input!";
}