Using document root as open basedir

Hello, I have one question concerning the "_SERVER" variables that we can find in the output of phpinfo(). For example: _SERVER["DOCUMENT_ROOT"] /var/www/user1 Is it possible to set this DOCUMENT_ROOT as open_basedir? This would be very comfortable, because I do not have to set this option in the Apache config. Does anyone has an idea how to change this in the PHP source? I would be very glad, if someone could help me. best regards, alex