$emailadres = strtolower($_POST['mail']);
$sql2 = 'SELECT emailadres FROM gebruikers WHERE emailadres="'.$emailadres.'"';
$check_mail = mysql_query($sql) or die (mysql_error());
$aantal_mail = mysql_num_rows($check_mail);
if ($aantal_mail !== 0) {
$error = "<p>Het E-mail adres is reeds geregistreerd</p>";
}