Description:
------------
There is no default for open_basedir(NULL). If there was a knob to turn on which would make DOCUMENT_ROOT to be the default then one could have open_basedir working for their current virtualhosts only by enabling the option.

I assume this is quite safe when used with safe_mode since safe_mode can disable the change of environment variables or PHP could read document root value from the vhost configuration.

I believe this would be trivial to implement and a very useful feature indeed.

Expected result:
----------------
I would like to be able to set open_basedir dynamicly without the need to set it per virtualhost seperately.

Actual result:
--------------
It is not possible to use any kind of variables in open_basedir setting so it is impossible to set it dynamicly.