Chris Jones has written up a guide for getting the Oracle Instant Client and PHP up and working on Linux and Windows.
Oracle 11g Instant Client is the easiest way to connect PHP to a remote Oracle database. This notes describes how to install PHP with the OCI8 Extension and Oracle Instant Client on Windows and Linux. The free The Underground PHP and Oracle Manual contains more detail and explains other installation options.
The guide walks you through enabling the PHP OCI8 Extension on Windows/Linux, verifying that its correctly installed and making a sample request to the remote Oracle database.