You brought this up before HTML and php versions of my site were live for about a week. Now lost all rankings
The worst google might think is you have duplicate content, that should not cause the kind of drop in SERP ranking that you’ve described.
When someone types or links to a website, they don’t usually use the full address.
https://www.example.com/index.html
A link to that site would normally be more like
Www.example.com
As I showed you on the previous post google hadn’t even indexed the PHP version of your site.
Now you may very well be in the search engine penalty box, but I don’t think it’s because of your PHP vs HTML pages.
IMO you’re “SEO expert” doesn’t Have a clue about SEO if they think changing industry standard relative navigation links to absolute URL’s would have any impact on your SERP ranking.