CREATE TABLE `country` (
  `id` int(11) NOT NULL auto_increment,
  `country` varchar(64) collate latin1_german2_ci NOT NULL default '-',
  `country_iso` char(5) collate latin1_german2_ci NOT NULL default '-',
  `country_code` varchar(12) collate latin1_german2_ci NOT NULL default '-',
  PRIMARY KEY  (`id`),
  KEY `country` (`country`),
  KEY `country_iso` (`country_iso`),
  KEY `country_code` (`country_code`)
);

INSERT INTO `country` (`id`, `country`, `country_iso`, `country_code`) VALUES (1, 'Deutschland', 'DE', '0049'),
(2, 'Österreich', 'A', '0043'),
...

<?php

if (!empty($_GET['country'])) {

$link = mysql_connect('localhost', 'staat', '3mu7Wz');

if (!$link) {
die('Verbindung nicht moeglich : ' . mysql_error());
}

$db_selected = mysql_select_db('staat', $link);
if (!$db_selected) {
die ('Kann Tabelle nicht benutzen : ' . mysql_error());
}

$result = mysql_query("SELECT country_code FROM country WHERE country_iso = ".$_GET['country']);
if (!$result) {
echo 'Abfrage konnte nicht ausgefuehrt werden: ' . mysql_error();
exit;
}
$row = mysql_fetch_row($result);

echo $row[0];

mysql_close($link);

}

?>

<tr>
    <td>  <label>Land:</label><select name="Staat" id="country">
          <option value="Deutschland" selected="selected">Deutschland</option>
<option value="Österreich">Österreich</option>
....
          </select>
  <br /></td></tr>
    <tr>
      <td><label>Landesvorwahl:</label>
   <input type="text" name="Landesvorwahl" id="country_code" value="" size="50"/></td>
     </tr>