Here is a collection of good PHP code libraries I somewhat use to create web based applications. This collection is about PHP only, in another article I will shortly tell you some good Python and Javascript/AJAX libraries.
Zend Framework
The Zend Framework is an object-oriented framework which is focusing on web 2.0 applications. It contains a lot of APIs to webservices like Google, Amazon, Yahoo!, Flickr.
CSV Utilities
PHP CSV Utilities or PCU, is a small, open source PHP library to simplify working with CSV files.
phpExcel
This project provides a set of classes, which allow you to write to and read from different file formats, like Excel 2007, PDF, HTML, … This project is built around Microsoft’s OpenXML standard and PHP.
phpQuery
phpQuery is a server-side, chainable, CSS3 selector driven Document Object Model (DOM) API based on jQuery JavaScript Library.
MC_Google_Visualization
MC_Google_Visualization provides simple support for integrating Google Visualization charts and graphs with your own internal database. It includes a complete parser for the Google Visualization Query Language, giving you the same ease of pivoting and formatting data from your database as is currently possible with Google Spreadsheets.
pChart
pChart is a PHP class oriented framework designed to create aliased charts.
FirePHP
FirePHP enables you to log to your Firebug Console using a simple PHP method call.
SimpleTest
The SimpleTest PHP unit tester is a PHP unit test and web test framework. It has support for SSL, forms, frames, proxies and basic authentication.
If you know any more good PHP libraries and tools, just leave a comment.