about the oepn_basedir

hi all, i just have a problem. we use apache for the web server and use mod_vhost_alias module for virtual hosting, and i also need some security strategy to forbid user1 in /vhost/user1 seeing details of /vhost/user2,so i decide to use open_basedir config. unfortunately there is no patterns in apache config file such as 'php_adminvalue open_basedir /vhost/%0' to make me set open_basedir dynamically. i know from php 5.3.0 on,open_basedir is set INI_ALL,so i can change it ,but there still some bugs under 5.3.0.so i still use 5.2.*. i decide to make a extension that when a request in ,it can change its open_basedir dynamatically. the quesition is what should i do?to develop a apache module or a php extension? i'm messed up.can someone help me,thanks a lot. best regards. lwice04