Ten years ago Andi Gutmans and Zeev Suraski started to renew the old PHP3 core and in 1999 PHP4 was published and became really famous among Web-Developers.
Bute 5 years late the successor was published and since then everyone is wondering why the era of PHP4 will end.
Today is the magical 8-8-8 where PHP4 is no longer supported so for everyone who still runs PHP4 for some reason
YOU SHOW NOW REALLY HAVE A LOOK AT THIS PAGE: www.php.net/manual/en/migration5.php
Detailed history of PHP can be found on www.php.net/history