//PSUEDO CODE
$res = mysql_query('SELECT DISTINCT `zip_code` FROM `users`');
while( $row = mysql_fetch_assoc($res) ) {
$pc1 = 'http://maps.google.com/maps/geo?q='.$row['zip_code'].',+UK&output=csv&sensor=false&key=ABQIAAAAcclaxnepdvvxx5D2cuoGw6G6HnB4VN4WqoMz7tmtKhTHGV82iKKTXvAg7YFpCFA6ptRc9g';
$data1 = file_get_contents($pc1);
$result1 = explode(",", $data1);
$lat = $result1[2];
$long = $result1[3];
mysql_query('UPDATE `users` SET `longitude` = \''.$long.'\',` latitude = \''.$lat.'\' WHERE `zip_code` = '.$row['zip_code']);
}
exactly as brad stated, select the zip codes, loop thru them, get the longitude/latitude in that loop, and do an update in the loop to add the longitude/latitude to the zip code you just looked up.