It's just based on the file extension of the URL and nothing more. No way to tell if it's actually PHP behind the scenes.

I see that page in my results too and the one you linked is post-redirect from an older domain that has the php3 URI scheme:

  [lammy@popola] curl -I http://www.terrier.com/trial/height.php3
  HTTP/1.1 301 Moved Permanently
  Date: Tue, 30 Apr 2024 20:51:07 GMT
  Content-Type: text/html; charset=iso-8859-1
  Content-Length: 258
  Connection: keep-alive
  Location: http://www.theRealJackRussell.com/trial/height.php

It is admittedly a little fuzzy since Google can just decide not to honor the `filetype:`/`ext:` operator like they do for so many others. I also included the `lr=lang_en` parameter which limits the results quite a bit, but I figured that would be more enjoyable as a submission to an English-language community like this one :)

Kagi's results are even better but I submitted Google instead due to the paywall: https://kagi.com/search?q=filetype%3Aphp3

The idea for this submission came from this xltronic-dot-com URL when I was looking up some old 808state stuff yesterday, a site which was not listed by either search engine: http://xltronic.com/mb/99108/faq.php3