Update: After re-reading the thread, it appears that I misunderstood the OPs requirements--I was allowing a timestamp to be updated that may be less than the maximum one in the table. The code should be more like:

$ST=$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);
[download]

ewhitt:

Let SQL do some of the work for you:

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

$ST->execute($hostname, $hostname, $timestamp, $hostname, $timestamp,
             $timestamp, $hostname);
[download]

Note: I would actually use a stored procedure so I wouldn't have to repeat the $hostname and $timestamp variables in an odd order. But I'm not a MySQL user, so I thought I'd do it with basic SQL.

This way, there's only one transaction with the database for each update: You just give the database a hostname and timestamp with a couple of decisions, and it'll do the rest.

Usual disclaimers apply: Untested code, et cetera.

...roboticus