With the release of another DLL driver for PHP on Windows to access Oracle databases, Chris Jones wanted to alleviate any confusion there might be between the two in a new blog post.
As I foreshadowed in a now distant post, Pierre has installed Oracle 11g Instant Client on the PHP Windows build box and updated the OCI8 build scripts for PHP 5.3 (and PHP 6). PHP 5.3 now has two mutually exclusive versions of the OCI8 extension DLL [...] The two DLLs have the same user script API that do the same thing. You only need to (and only can) enable one at any one time. Choose the one that matches your Oracle client libraries.
The two DLLs - php_oci8.dll and php_oci8_11h.dll - have pretty self-explanitory names, but it could be confusing as to whether you need both or not if you go the 11g route.