PHP History (From Php.net)

PHP/FI:
PHP/FI was created by Rasmus Lerdorf in 1995, initially as a simple set of Perl scripts for tracking accesses to his online resume. He named this set of scripts 'Personal Home Page Tools'.
Its Stands for Personal Home Page / Forms Interpreter.

PHP/FI 2.0:

PHP/FI 2.0 was officially released only in November 1997, after spending most of its life in beta releases.the second write-up of the C implementation, had a cult of several thousand users around the world (estimated), with approximately 50,000 domains reporting as having it installed, accounting for about 1% of the domains on the Internet

PHP 3.0:

PHP 3.0 was the first version that closely resembles PHP as we know it today. It was created by Andi Gutmans and Zeev Suraski in 1997 as a complete rewrite, after they found PHP/FI 2.0 severely underpowered for developing an eCommerce application they were working on for a University project. In an effort to cooperate and start building upon PHP/FI's existing user-base, Andi, Rasmus and Zeev decided to cooperate and announce PHP 3.0 as the official successor of PHP/FI 2.0.

By the end of 1998, PHP grew to an install base of tens of thousands of users (estimated) and hundreds of thousands of Web sites reporting it installed. At its peak, PHP 3.0 was installed on approximately 10% of the Web servers on the Internet.

PHP 4:

By the winter of 1998, shortly after PHP 3.0 was officially released, Andi Gutmans and Zeev Suraski had begun working on a rewrite of PHP's core. The design goals were to improve performance of complex applications, and improve the modularity of PHP's code base. Such applications were made possible by PHP 3.0's new features and support for a wide variety of third party databases and APIs, but PHP 3.0 was not designed to handle such complex applications efficiently.

The new engine, dubbed 'Zend Engine' (comprised of their first names, Zeev and Andi), met these design goals successfully, and was first introduced in mid 1999. PHP 4.0, based on this engine, and coupled with a wide range of additional new features, was officially released in May 2000, almost two years after its predecessor, PHP 3.0. In addition to the highly improved performance of this version, PHP 4.0 included other key features such as support for many more Web servers, HTTP sessions, output buffering, more secure ways of handling user input and several new language constructs.

Today, PHP is being used by hundreds of thousands of developers (estimated), and several million sites report as having it installed, which accounts for over 20% of the domains on the Internet.

PHP 5

PHP 5 was released in July 2004 after long development and several pre-releases. It is mainly driven by its core, the Zend Engine 2.0 with a new object model and dozens of other new features.