Call to undefined function oci_connect()

Hi experts,

I am getting the following error when trying to connect to my Oracle Database via PHP using OCI8:

 Call to undefined function oci_connect()

Here is what I have tried:
- uncommented "extension=php_oci8_11g.dll" and restarted XAMPP server.
- listed "C:\app\Administrator\product\11.2.0\client_1\bin" first in my environment variables and restarted server.
- made sure php_oci8_11g.dll existed in C:\xampp\php\ext.
- oci8 is not listed in my phpinfo().
- php log: PHP Warning:  PHP Startup: Unable to load dynamic library 'C:\xampp\php\ext\php_oci8_11g.dll' - The specified procedure could not be found.

Here is my environment:
- Windows Server 2008 R2 x64
- Oracle Client 11g R2 (successfully connects to my Oracle db via SQL Plus)
- XAMPP 1.8.3 (PHP 5.5.3)

What could I be missing?

Thank you in advance.