change the php global environment variable

Hi,

I am facing a small problem of how to change the PHP global variable such as $_ENV['INFORMIXSERVER']. The problem is I had installed the PDO for informix database with a name called 'a' for suppose, and later changed the INFORMIXSERVER to 'b' for suppose for the informix DBMS due to some technical reasons. Now, the $_ENV['INFORMIXSERVER'] is still showing me the name 'a'. How to change this so that I could execute some executables of informix can be run through PHP (shell_exec). Is there any means to change the global PHP environment variables permanently. If so, Please guide me where exactly these values are stored and how to change them.

Thanks in advance.

Regards
Kiran.