Answer:
PHP began as a set of perl scripts to track access to his resume. He called it "Personal Home Page Tools". When he needed more tools, he wrote a C implementation, made made it public. He then changed the name to "Personal Home Page / Forms Interpreter", or PHP/FI.
PHP/FI 2 was spend mostly in beta releases, and about 1% of the servers where using it.
PHP 3 followed closely. 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. The whole new language was released under a new name, that removed the implication of limited personal use that the PHP/FI 2.0 name held. It was named plain 'PHP', with the meaning being a recursive acronym - PHP: Hypertext Preprocessor.
PHP 4 includes a new engine, the Zend engine. It was the first widely known release of php, and is still the most used today. It included many enhancement, such assupport for many more Web servers, HTTP sessions, output buffering, etc.
PHP 5 is the most recent version, and it is driven by the Zend Engine 2.0. The new features include a brand new object model, namespaces, and multiple other improvements.