Как то так.

$login = filtr($_POST[\'login\']);
$pass = filtr($_POST[\'pass\']);

if (isset($_POST[\'reg\'])) {
if (empty($login) {
$err[] = \'ERROR LOGIN\';
}
if (empty($pass) {
$err[] = \'ERROR PASS\';
}

if (!$err) {
mysql_query(\"INSERT INTO `users`(`login`,`pass`)VALUES(\'\".$login.\"\',\'\".$pass.\"\')\";
}
} else {
foreach($err AS $err_info) {
echo $err_info;
}

echo \'<form action=\"\" method=\"post\">
Логин<br><input name=\"login\"/><br>
Пароль<br><input name=\"pass\"/><input type=\"submit\" name=\"reg\" value=\"ВВОД\"/></form>\';