We don't know the exact date, maybe there is none, maybe it's not really important at all. But i think it's notable that the web programming language for pretty homepages is already 20 years old. A lot of people hate PHP, some take it seriously, we love it.
Credit should go to the really nice history of PHP project by Yusuke Ando and Tsutomu Kawamura. This is actually just a remix. Is there an error or something important missing? Please tell us!