Zeev Suraski and Andi Gutmans rewrote the parser in 1997 and formed the base of PHP 3, changing the language's name to the recursive acronym PHP: Hypertext Preprocessor.[5][19] Afterwards, public testing of PHP 3 began, and the official launch came in June 1998. Suraski and Gutmans then started a new rewrite of PHP's core, producing the Zend Engine in 1999.[20] They also founded Zend Technologies in Ramat Gan, Israel.[5]
On May 22, 2000, PHP 4, powered by the Zend Engine 1.0, was released.[5]
As of August 2008 this branch reached version 4.4.9. PHP 4 is no longer under development nor will any security updates be released.
https://en.wikipedia.org/wiki/PHP#PHP_3_and_4