If you want to specify a connection timeout in case there is network problem, you can edit the client side (e.g. PHP side) sqlnet.ora file and set SQLNET.OUTBOUND_CONNECT_TIMEOUT. This sets the upper time limit for establishing a connection right through to the DB, including the time for attempts to connect to other services. It is available from Oracle 10.2.0.3 onwards.
In Oracle 11.1, a slightly lighter-weight solution TCP.CONNECT_TIMEOUT was introduced. It also is a sqlnet.ora parameter. It bounds just the TCP connection establishment time, which is mostly where connection problem are seen.
The client sqlnet.ora file should be put in the same directory as the tnsnames.ora file.
