This lengthy article by Zeev Suraski includes a brief history of PHP, explanations of where they went wrong in previous versions, the design changes made in PHP 4, and more.
Here is a compilation of tips on how to optimize Apache on Unix for PHP and CGI programs. The tips can also be applied to Perl and Python.
This article shows that PHP and MySQL are both mature and widespread technologies that can be considered a primary web-application platform, not just an alternative to ASP, JSP, Cold Fusion and a host of other web-app scripting solutions out there.
This short comparison by John Lim explains what he likes about PHP vs ASP and why his company is considering switching their language of choice.
A compilation of all available PHP debuggers and caches, with benchmarks.
This review by Paul Ferris of PHP 4 appeared on LinuxPlanet and explains why in the author's opinion version 4.0 is a vast improvement, which he backs up with
This article that appeared at internet.com includes a general overview, a review of the architecture and functionality and finishes off with a list of pro's and con's about the language.
After experiencing a performance decrease under heavy load using ASP, this developer explains their migration to PHP and how it allowed for a more elegant solution.
Several newsgroup postings that outline the pros and cons of Cold Fusion and PHP and compare them on Platform, Language, Database, File-System Support and more.
Compares Perl and PHP for building Web sites based on ease of learning, code maintenance, speed and maturity. | 
