[Marco van Herwaarden]

The first i would suggest is to read up on the articles about creating secure scripts (see the articles section).

1 example of vulnerable code:

PHP Code:

$WebsiteUsername = $_POST['Website_Username'];
.......
if (isset($WebsiteUsername) && $WebsiteUsername != NULL && $UserVerify == "Verify_That")
{
    $CustomerInfoSQL = "SELECT username FROM user WHERE username='$WebsiteUsername'"; 


$WebsiteUsername is uncleaned user input. Should at least escaped before used in a query.