This great collection of classes was in my eyes the first real useful library which delivered the solution to most of the basic / standard problems in PHP based software projects. [...] The easy implementation of DB-abstraction, template engine, authentication, permissions, session management and others made it easy to handle these problems in a standardized way. The strict object oriented code gave the developer the necessary flexibility to customize and extend the code where he/she needs it.

This year is (about) the ten year mark for the existence of the language and several commentors on the post remember the "good ole days" of PHP3 sessions, permissions and even the first version of Zend's website.