1. When the documentation says "page" it really means "file" (ie, __FILE__). So
>Gets the time of the last modification of the current file.

2. PHP can't know what files may be include()d later on, but you can use 
get_included_files() and filemtime() a la the comment by Moro [1] to get the 
most recent modification date of the ones include()d so far.
IMO a feature request to bundle this into getlastmod() seems quite reasonable 
but you might want to make a separate feature request ticket so this one can 
remain a bug report.

[1] http://php.net/function.getlastmod#109250

Ok, then what I'd suggest is that

"time of the last modification of the current page"

be changed to

"time of the last modification of the current script file" 

in both the "Description" and "Return values" section of the doc page.

Additionally it should note that this is always the modification time of the top-level script file, even if called from within an include file; modification times of include files don't count.

I'll log a separate feature request asking that functionality be added to getlastmod() to check included file modification dates also.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

getlastmod either executes a SAPI handler or references a global stat cache for PATH_TRANSLATED, it does not have anything to do with __FILE__.

Documentation updated for clarity.