registro.php

<?php
/**
 * Hola
 */

$connection = [
    'host'       => '',
    'username'   => '',
    'password'   => '',
    'database'   => ''
];
// HOW CAN I RESIST YOU

list($host, $username, $password, $database) = $connection;

$mysqli = new mysqli($host, $username, $password, $database);

if ($mysqli->error) {
    exit; // how do I handle this
}

/*
700 = user taken
710 = 5 accounts on email
600 = custom ban message
*/

$get_val = function ($val) use ($mysqli)
{
    if (! isset($_POST[$val])) {
        return '';
    }
    $val = $_POST[$val];
    $val = trim($val);
    return $val;
};

$username  = $get_val('n');
$password  = $get_val('p');
$email     = $get_val('email');

if ($_SERVER['HTTP_REFERER'] === 'http://clubpenguin.io/penguin/chat.swf') {
    exit('e=600');
}

if ( ! ($username && $password && $email)) { // empty?
    exit('e=28');
}

if (! preg_match('/^[a-z0-9]{3-14}$/', $username)) {
    exit('e=600');
}

if (! filter_var($email, FILTER_VALIDATE_EMAIL)) {
    exit('e=28');
}

if (mb_strlen($password, 'utf-8') < 3) {
    exit('e=28'); // too short
}

$user_exists = "SELECT COUNT(*) AS size FROM users WHERE username = '%s'";
$user_exists = sprintf($user_exists, $mysqli->real_escape($username));
$user_exists = $mysqli->query($user_exists);
if (! $user_exists) {
    // query failed
    exit('e=28');
}
$user_exists        = $mysqli->fetch_row();
$user_exists = !! $user_exists[0];

if ($user_exists) {
    exit('e=28');
}

$email_exists = "SELECT COUNT(*) AS size FROM users WHERE username = '%s'";
$email_exists = sprintf($email_exists, $mysqli->real_escape($email));
$email_exists = $mysqli->query($email_exists);
if (! $email_exists) {
    // query failed
    exit('e=28');
}
$email_exists = $mysqli->fetch_row();
$email_exists = !! $email_exists[0];

if ($email_exists) {
    exit('e=29');
}

$password = strtoupper(md5($password));

$insert = [
    'Username'       => $username,
    'Password'       => $password,
    'Email'          => $email,
    'RegisteredTime' => time(),
    'Color'          => '1',
];

$columns = array_keys($insert);
$values  = array_values($insert);

$query = 'INSERT INTO users (`' . implode('`', $columns) . "`) VALUES ('" .
         implode("'", $values) . "')";

$insert = $mysqli->query($query);

if (! $insert) {
    exit('e=28');
}
exit('e=0');