How is Google determining that these pages are PHP3?
This is one of the pages from the SERPs:
https://www.therealjackrussell.com/trial/height.php
But there is nothing in the page or its HTTP headers that would tell you the page is even PHP (except for the extension which might be misleading...)
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:
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