Using the Xero API with PHP
The XeroOAuth-PHP library can be used with all application types – public, private and partner, and is currently the only actively maintained library.
KohanaXero wrapper for the Kohana framework (private applications only).
Private Applications – PHP-Xero
David Pitman has contributed a library for use with private applications.
This library is no longer actively maintained as of June 1st 2013