Most of the keywords listed on the following URL are now highlighted:
  http://php.net/manual/en/reserved.keywords.php

Also color single-quoted strings, and require that //-type comments
are preceded by whitespace or are alone on a line.

With-feedback-from: M <taur@mail.com>

(And in the bargain sort or group some of the keywords.)

This addresses https://savannah.gnu.org/patch/?9342

.
Suggested-by: M <taur@mail.com>

Some .rdf files are XML, but the .rdf extension seems to be used
also for other things.  So... use the 'header' mechanism.

(Hadn't seen that one because of my additional bindings.)

And because I think it looks slightly better.

This avoids applying the default syntax, or the syntax specified
with --syntax, to a ^G help text.

This addresses a followup of https://savannah.gnu.org/bugs/?50362

.
Reported-by: Mike Frysinger <vapier@gentoo.org>

A tag begins not merely with a "<" but it must be followed by an
ASCII alphabetic character or one of "/", "!" and "?".

Further, color all the valid attribute names in red.

They are not documentation, they are functional elements of nano.