<?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!";
}

}
?>

<?php
echo $output;
?>