The culprit is the too restrictive fix for bug #71536, which prevents
`php_libxml_streams_IO_write()` from properly executing when unclean
shutdown is flagged.  A *more* suitable solution is to move the
`xmlwriter_free_resource_ptr()` call from the `free_obj` handler to an
added `dtor_obj` handler, to avoid to write to a closed stream in case
of late object freeing.  This makes the `EG(active)` guard superfluous.

We also fix bug79029.phpt which has to use different variables for the
three parts to actually check the original shutdown issue.

Thanks to bwoebi and daverandom for helping to investigate this issue.

We backport the fix PHP 7.3, since this branch is affected as well.

(cherry picked from commit b5e004379647bd1ebb75eb2eac8826fb6abdd3d8)
(cherry picked from commit e36daa6927c05d2e687bb77495ef206cde118b33)
(cherry picked from commit 2704ee6844c03348de9d15e74646d09007ef0f7c)

Even if we don't initialize the callback on every request, we
should still reset our globals to default values...

* PHP-7.2:
  Fixed bug #76777 and added test

* PHP-7.1:
  Fixed bug #76777 and added test

Set undefined values to null rather than undefined.

The $Id$ keywords were used in Subversion where they can be substituted
with filename, last revision number change, last changed date, and last
user who changed it.

In Git this functionality is different and can be done with Git attribute
ident. These need to be defined manually for each file in the
.gitattributes file and are afterwards replaced with 40-character
hexadecimal blob object name which is based only on the particular file
contents.

This patch simplifies handling of $Id$ keywords by removing them since
they are not used anymore.