I work on an open source software that runs on PHP. We collect anonymized data to be able to do some statistics about execution environment of our software, and I figured out that the PHP internal server information was not collected as expected. This is due to the fact that we expect the SERVER_SOFTWARE to be compliant with RFC3875 (see below).

I am not sure this RFC should apply here, but as far as I know, all webservers I know are compliant with it.
Examples:

• Apache/2.4.56 (Debian)
• nginx/1.14.0
• lighttpd/1.4.53
• Microsoft-IIS/10.0
• Microsoft-HTTPAPI/2.0

According to RFC3875

The SERVER_SOFTWARE meta-variable MUST be set to the name and version
of the information server software making the CGI request (and
running the gateway). It SHOULD be the same as the server
description reported to the client, if any.

 SERVER_SOFTWARE = 1*( product | comment )
 product = token [ "/" product-version ]
 product-version = token
 comment = "(" *( ctext | comment ) ")"
 ctext = <any TEXT excluding "(" and ")">