A list of some very useful PHP resources.
If you are new to PHP, this probably the best place to start.
ADOdb is a database abstraction PHP library designed for speed extensive portability. Supports active records, pagination, object-relational mapping and database based sessions.
Full featured email class for PHP to simplify you email sending tasks. Supports HTML mails, attachments, SMTP authentication and much more.
A lightweight MVC library. If you have never implemented a MVC framework before, CodeIgiter is your best bet. Easy to learn. Excellent documentation. Best for small to medium scale projects.
The heavyweight of PHP frameworks. Steep learning curve but has lot of libraries and is best suited for large projects. You can also use Zend framework libraries in other frameworks (Such as CodeIgniter)
A powerful and open source newsletter script.
PHP based ACL script to add a fine grained and powerful permissions system to your application.
PHP encoder with support for bytecode compilation to protect your PHP scripts from theft and piracy. Much cheaper than similar product from Zend. Also offers online encoding which is a great tool for those who only need to encrypt scripts occasionally.