Ok,
forget about the environment variables, since you're using the instant client. Instead use the easy connect naming method with oci_connect, that is something like this:
oci_connect("user", "pass", "//dbhost[:port]/servicename");
dbhost is the hostname or ip address of the oracle database server
: port is optional if your installation of oracle uses another port than the standard 1521 one
servicename ist the name of the oracle service you want to connect to.
I didn't use the easy connect naming method myself so far since I always had a complete oracle client installation including the network/admin directory with tnsnames.ora and sqlnet.ora. But it should work.
Thomas