Just check 2.6 documentation, the issue is quite clear : It's empty.
It's not clear from the ticket what the issue is, but here's what I suspect:
The current left-hand navigation has a limit in that it only shows one sub-component at a time, and users must click Main Menu to discover the other components.
PHP docs are available in all components - see for example:
We are working to improve that nav, and on the assumption that this is the cause of the issue raised I will keep this ticket open until work is finished.
If the cause of complaint is actually a specific page not in the 3.0 PHP docs, please comment here.
Closing on assumption that fixed by nav updates - see above comment.