Do you have a phpinfo.php you can look at? If you don't, you can make one, it's just this:
Code:
<?php
phpinfo();
?>
in a file named phpnfo.php (or I guess whatever you want to call it), then put that in public_html and point your browser at it. I'm not an expert on all the info in there, but it might tell you something if you search it for open_basedir, and maybe look at what php.ini it's loading.