Chris Jones has a recommendation for those compiling Oracle support into PHP - there's a simple way to reduce the amount of disk space that your Instant Client install uses - a "only what's needed" setup.

Most PHP OCI8 users link PHP with the Oracle Instant Client (IC) libraries that handle the underlying "client" (i.e PHP OCI8 extension) communication with the database. IC is relatively small for the features it gives, and is very easy to install. A ZIP or RPM download unpacks a few libraries. [...] Although IC isn't particularly big - and it's getting relatively smaller all the time - there are two ways to reduce its size.

His two recommendations are to use the lite client version instead of the full-blown one (only has common character sets) or you can just remove the JDBC/ODBC/OCCI libraries from the full install. He lists out the files that are actually needed by the OCI8 extension to compile correctly.