Losing the ability to connect to Oracle database

I recently upgraded to PHP5 and now have a curious Oracle problem.

The PHP scripts query the database fine.  Then overnight, the database
shuts down for backup and restarts.  The next day, PHP can no longer
connect to the database until I restart Apache.  Then, it's happy until
Oracle restarts the next night.

We did not encounter this problem with PHP4.

I've tried adjusting the php command from the old ocilogon() to
oci_connect() but the problem continues.  Incidentally, I am using a
simple connection, not a persistent one.

Can someone advise what's wrong here or how to get PHP/Apache to survive
a database restart.