http_response_code() depends on the SAPI (Apache or FPM+nginx) supporting the code. nginx doesn't mind a custom 210 but Apache does and it (not PHP) substitutes a 500 instead [1].

If you want a custom status then use header(), which Apache handles differently than if you just give the code.

header("HTTP/1.1 210 Whatever That Code Means");


[1] They support 200 OK through 208 Already Reported. Anything 2xx above that will be rejected. See their modules/http/http_protocol.c for more.
https://svn.apache.org/viewvc/httpd/httpd/branches/2.4.x/modules/http/http_protocol.c?revision=1799890

Thank you for the clarification. It might be a good idea to mention this in the documentation; something like "Available response codes depend on the server software used". There's a comment about that (http://php.net/manual/en/function.http-response-code.php#116880), but this is more important than that.

#116880 is partially correct in that PHP does use that list, but it's only for php-fpm, the built-in dev server, and when running as regular CGI (as far as I can see).

A note along the lines of
  http://php.net/manual/en/function.http-response-code.php#114996
that support depends on the SAPI and how to use header() as a workaround for non-standard codes would be a good idea.