BackPress is a PHP library of core functionality for web applications. It grew out of the immensely popular WordPress project, and is also the core of the bbPress and GlotPress sister-projects.

It includes a variety of the foundations you need to build robust, scalable web applications including (amongst other things):

  • Logging,
  • User Roles and Capabilities (Permission systems),
  • Database connections (across multiple servers and multiple datacenters),
  • HTTP Transactions,
  • XML-RPC Server and Client,
  • Object caching,
  • Formatting,
  • XSS and SQL injection protection, including a variety of powerful escaping functions,
  • Taxonomies and
  • Options management

And the best part? It’s all licensed under a generous GPL2 license, so you can use it in your own free or commercial projects.