[urga]

$error_messages = array();
if ($error_message1) {
 $error_messages[] = $error_message1;
}
if ($error_message2) {
 $error_messages[] = $error_message2;
}

echo implode ('<br>', $error_messages);

oder

$error_messages[] = $error_message1;
$error_messages[] = $error_message2;
echo implode ('<br>', array_filter ($error_messages));