It sounds like the Oracle client libraries that PHP is calling are taking a long time to resolve the database service name into an actual machine to connect to. It should eventually time out and return an error message (example: ORA-12154 - Could not resolve service name).
Can you connect to the Oracle database using 'sqlplus username/password@servicename' from that userID on that machine?
Also try running your PHP script from the command line so that you can see any other errors when PHP might be encountering.
-- Michael
Darkstreak Consulting
www.darkstreak.com