NOTE: If you're upgrading to 6.x-1.0, and you haven't updated in a while, you may need to go back into your view and select an include/exclude option in your PHP filter; otherwise you might get unintended results for the view.
Based on Views issue #115947. The module adds a filter to your Views which can dynamically select a set of nodes to include or exclude from your filter chain by way of PHP snippets. Now your views can be customized in whatever way you want, without being limited to the existing filters and without having to write a new filter for functionality you may only have to use once.
Some basic documentation is in the README file; some illustrative examples can be found in the Views PHP Filter section in the handbook.
A note on PHP troubleshooting:
Using raw PHP to filter can be a powerful tool, but it does presuppose certain things -- most notably, a solid understanding of PHP and a working knowledge of Drupal internals. Sadly, I can't address either of these in the issue queue, or I'd be spending all my time answering "how do you do XYZ with a PHP snippet?" questions. I will be happy to address issues dealing specifically with this module, but "how do you...?" questions are specific to your site, not to this module, and don't belong in the module issue queue and won't be addressed here. There are other resources which are more appropriate for troubleshooting your code. If you need help with your PHP code, you may want to try the forums or IRC; if you figure out some clever way to do something with a PHP snippet that you'd like to share with others, you may want to share it on the handbook page.