php - Which HTTP response code to send on unhandled request -

i have php application has router should have 2 default handlers 401 unauthorized , 404 not found.

but happen no handlers set 2 cases. in case, need have default response tells request unhandled, have set response code 400 bad request, feel not appropriate.

what response code should use unhandled request?

4xx client's problem. 5xx (server-side) problem. if cannot handle request - http 5xx.

in case looks 501 not implemented best fit.

references:

• http://www.restapitutorial.com/httpstatuscodes.html
• https://en.wikipedia.org/wiki/list_of_http_status_codes#5xx_server_error