The library supports PHP versions 4.3.2 and up and is tested on several versions; for a complete list, check the PHP Buildbot. We support all of these PHP versions because there is a wide distribution of PHP versions that are in active use across the Internet. For instance, the nexen.net statistics for May 2008 show about 20% of PHP installations running PHP 4.3, almost 30% running PHP 5.2 and about 40% running PHP 4.4.
Most of the library testing is done on Linux systems, although the tests are also run on Windows. If you have a platform on which you'd like the tests to run to ensure continuted compatibility and you're willing to host the testing environment, let us know.
If you are seeing a lot of warnings in your browser or in your server log when using the library, it is likely that you're running PHP 5.
Since the library is built to support both PHP 4 and PHP 5, some PHP 4 code will generate warnings when run on a PHP 5 interpreter. The warnings can be safely ignored because the library is tested extensively on PHP 5. If you are seeing the warnings in a web browser, you can configure PHP to log them to a more appropriate place. See the PHP logging configuration documentation for more information, in particular the display_errors and error_log php.ini settings.
If you are not running PHP 5, please let us know about the warnings so they can be addressed.
Once PHP 4 is no longer necessary for popular OpenID-enabled software such as MediaWiki, PHPBB, and WordPress, the library can be ported to pure (warningless) PHP 5.