[RCU] temp directory

I am looking at my old test setup on Centos6, and the tempdir is set to:

main.inc.php:$rcmail_config['temp_dir'] = '${_tmppath}';

I can't find where the variable tmppath is defined though at a guess it is some Centos apache or php default temp path?

I see that the main.inc.php is gone and the format now is:

config.inc.php:$config['temp_dir'] = '<your temp directory here>';

So I tried:

config.inc.php:$config['temp_dir'] = '${_tmppath}';

And of course this did not work. I was not supprised.

How might I figure out how tmppath is defined, or where is a better place for temp files than under /usr/share? Like /tmp/?

thanks