On Windows platforms, the php_pdo_oci8.dll library was built against the Oracle8i and 9i library. php_pdo_oci.dll was built against the Oracle
10g library. So, php_pdo_oci8.dll may be used with Oracle 8i or 9i clients, and php_pdo_oci.dll should be used to connect to Oracle 10g clients.
(Solution found at http://bugs.php.net/bug.php?id=41676)
If both libraries are installed, this would likely cause problems
connecting to Oracle.