Have you tried taking a look at your phpinfo page?
If you don't have one, just open a new txt file, put this in there.
Save it as a php file and ftp it up to your html dir.
Then point your browser to
www.yoursite.com/phpinfo.php
It might show you where the problem lies, if you don't have access to your ini file, you'll know better what to ask your host to do.