PHP-Frame is one of my oldest projects, dating back to 2005. It is a set of minimalist PHP frameworks. Frameworks is plural because, while PHP-Frame can be used as a coherent package, each of the modules stands alone.
Each of these frameworks were developed with simplicity, flexibility and performance in mind. The code is simple enough that if a project has special needs, it is trivial to customize the authentication or template system.
The frameworks I find most valuable: the CortexTemplate templating engine, security.php for cleansing input, and the MySQL database layer. I have also used the authentication routines in dozens of sites.