Store session files in different directories in PHP

I have several sites hosted on the same server, in different directories (e.g: server.com/site_one, server.com/site_two) and I want to store the session files for each site in a different directory.
How do I do that?

Thanks!

If you're running PHP as an Apache2 module, the following should work for you.

In the folder you want to change the session path (e.g site.com/site_one), create a .htaccess file with the following in it:
php_value session.save_path "/path/to/separate/sessions/site_one".

Repeat this and change the directory for each folder you want to have a separate session path.