How to enable oci in xampp 5.6.3?

Well, I've installed XAMPP 1.8.2, PHP 5.4 and Apache 2.0, and I use php_oci8_11g.dll and works fine.

You are trying use php_oci8_12g.dll, I recommend you read this link http://www.oracle.com/technetwork/articles/dsl/technote-php-instant-12c-2088811.html with especifications for run oci12g.