Changes have been made in Git (master):

commit f41dc10b1a23e0f16e7aca04aa20b1df4d43b3f0
Author: Jan Schneider <jan@horde.org>
Date:   Mon Feb 1 17:31:07 2016 +0100

     Use configured temporary directory (Request #11756).

  .../Core/lib/Horde/Core/Factory/MimeViewer.php     |    1 +
  .../Mime_Viewer/lib/Horde/Mime/Viewer/Ooo.php      |    7 +++----
  framework/Mime_Viewer/package.xml                  |    2 ++
  3 files changed, 6 insertions(+), 4 deletions(-)

http://github.com/horde/horde/commit/f41dc10b1a23e0f16e7aca04aa20b1df4d43b3f0

How can it configured for cases like:

/usr/share/pear/Horde/Mime/Viewer/Ooo.php:            $tmpdir = 
Horde_Util::createTempDir(true);

----

     static public function createTempDir($delete = true, $temp_dir = null)
     {
         if (is_null($temp_dir)) {
             $temp_dir = sys_get_temp_dir();
         }

Configure the temp directory in horde's configuration.

there is an upstream php report which is open for more than 4 years: 
https://bugs.php.net/bug.php?id=44972

So, do not wait on PHP to fix this...

Horde places files with hardcoded names (e.g. 'horde_cache_gc' or 
'passwd.lock') into the directory returned by sys_get_temp_dir().   
Unfortunately, this function is broken by returning always the 
world-writable /tmp directory and there is no way to change this (I am 
aware of modifying the TMPDIR environment variable, but this requires 
changes in the apache startup script affecting the whole apache server 
inclusive unrelated vhosts).

Having cache data in /tmp requires to add this directory to 
'open_basedir' which opens vectors for other security holes.

There are functional problems too when different vhosts with perhaps 
different horde versions are accessing the same /tmp/horde_cache_gc 
file.

You should replace all occurrences of sys_get_temp_dir() with a 
utility function which returns e.g.

| getenv('TMPDIR') ? getenv('TMPDIR') : sys_get_temp_dir()

So, TMPDIR can be configured per vhost/directory an the apache configuration.