5017 reg page?

<?php
/*
 * Generic Register Page
 * Author: Felipe Vieira Vendramini
 * Configuration
 */
$MySql["Host"] = "localhost"; // host, duuh
$MySql["User"] = "root"; // Username
$MySql["Pass"] = "test"; // Password
$MySql["Data"] = "zf"; // Database

$Database = new PDO('mysql:host='.$MySql["Host"].';dbname='.$MySql["Data"].';charset=utf8', $MySql["User"], $MySql["Pass"], array(PDO::ATTR_EMULATE_PREPARES => false, PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION, PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'));
$Database->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

/*
 * Common Functions
 */
function CheckIfNegative($Number)
{
    if($Number < 0)
    {
        $Number *= (-1);
    }
    return $Number;
}

function CheckINT($Number)
{
    if(filter_var($Number, FILTER_VALIDATE_INT) === false)
    {
        return false;
    }
    return true;
}

function ValidateUser($string){
    $expression = "[A-Za-z0-9]";
    if(preg_match($expression, $string)){
        return true;
    }
    return false;
}

function ValidateEmail($string){
    $string = strtolower($string);
    $expression = "/([\w\-]+\@[\w\-]+\.[\w\-]+)/";
    if(preg_match($expression, $string)){
        return true;
    }
    return false;
}

/*
 * Form validation
 * Fill:
 * Username: varchar(16)
 * Password: varchar(16)
 * SecurityCode: bigint(8)
 * Email: varchar(64)
 */

if(!ValidateUser($_POST["Username"]) || !ValidateUser($_POST["Password"])){
    //Handle Invalid Username or Password
}

$Username = $_POST["Username"];
$Password = $_POST["Password"];

if(strlen($_POST["Username"]) < 6 || strlen($_POST["Username"]) > 16){
    //Handle Username Lenght Error
}

if(strlen($_POST["Password"]) < 6 || strlen($_POST["Password"]) > 16){
    //Handle Password Lenght Error
}

$CheckName = $Database->prepare("SELECT username FROM `account` WHERE `username`=:name");
$CheckName->bindValue(':name', $Username, PDO::PARAM_STR);
$CheckName->execute();
$CountN = $CheckName->rowCount();

if($CountN > 0){
    //Handle Existing Username
}

if(!ValidateEmail($_POST["Email"])){
    //Handle Invalid E-mail
}

$Email = $_POST["Email"];

$CheckMail = $Database->prepare("SELECT email FROM `account` WHERE `email`=:mail");
$CheckMail->bindValue(':mail', $Email, PDO::PARAM_STR);
$CheckMail->execute();
$CountM = $CheckMail->rowCount();

if($CountM > 0){
    //Handle Existing Mail
}

$SecurityCode = CheckIfNegative($_POST["SecurityCode"]);

if(!CheckINT($SecurityCode)){
    //Handle Invalid format Security Code
}

if($SecurityCode < 10000000){
    //Handle 8 Digits Security Code Error
}

$InsertRow = $Database->prepare("INSERT INTO account (`username`,`password`,`email`,`code`) VALUES (:name, :pass, :mail, :code)");
$InsertRow->bindValue(':name', $Username, PDO::PARAM_STR);
$InsertRow->bindValue(':pass', $Password, PDO::PARAM_STR);
$InsertRow->bindValue(':mail', $Email, PDO::PARAM_STR);
$InsertRow->bindValue(':code', $SecurityCode, PDO::PARAM_INT);
$Check = $InsertRow->execute();

if(!$Check){
    //Handle Insert failed
}

die("OK");