I'm having exactly the same problem and I'm looking for an answer myself, but for the time being - here's a work-around: Run your scripts via cgi, instead of mod_php.
Create an .htaccess file in the root folder (of your site) and put the line
AddType php-cgi php
That should disable the open_basedir restriction (instead of adjusting it). As I said, it's a just a work-around, not a solution.
Please contact me, if you find something better !!! I have a nagging suspicion this might be a PHP-bug ...