$ST=$DBH->prepare(<<EOSQL);
if not exists (select * from tbl where timestamp > ?)
   if exists (select * from tbl where hostname=?)
      update tbl set timestamp=? where hostname=?
   else
      insert tbl (hostname, timestamp) values (?, ?)
EOSQL

$ST->execute($timestamp, $hostname, $timestamp, $hostname, $hostname, $timestamp);