Código PHP:
$selected_id = $_POST['domain_id']; // Para realizar la comparación
while($value = mysql_fetch_array($result))
{
$domain_id=$value['domain_id'];
$keep = (R4crypt($domain_id) == R4crypt($selected_id)) ? ' selected="selected" ' : '';
$domain_description=substr($value['domain_description'],0,70);
$option_line=$option_line.'<option value="'.R4crypt($domain_id).'"$keep>'.$domain_descript ion.'</option>'."\n";
if ($domain_id==$_POST['domain_id'])
$option_line=$option_line.'<option selected value="'.R4crypt($domain_id).'">'.$domain_descript ion.'</option>'."\n";
Del require_once no pusiste el error, así que no sé.