I would send the page to itself and refill all their other input (using something like:<input type="text" name="CountryPhoneCode" size="6" maxlength="5" <?php echo "value='$countryphonecode'"; ?>>) then do a database search for the country, select the CountryPhoneCode and in that field add some thing like the following...
<input type="text" name="CountryPhoneCode" size="6" maxlength="5" <?php echo "value='$countryphonecode'"; ?>>
Otherwise you're looking at doing some JavaScript thing..