Spl_autoload_register not working in production

Question

I have a WampServer on my dev machine and use a LAMP stack for production (L = Ubuntu 12.04 LTS). PHP version is 5.3.10-1 on LAMP and 5.4.3 on WampServer.
As far as I can tell the spl_autoload_register function should work on php 5.3 + and so should namespacing which I use for a custom library. So I don't know why I keep getting the following error:
"Fatal error: Class 'Sb\Bootstrap' not found in [path to index.php]"
This only happens on the production machine. I have made sure that the library directory is included in the include path using get_include_path().
I am using code like the following:
define('LIBRARY_PATH', dirname(__DIR__));
set_include_path(get_include_path() . PATH_SEPARATOR . LIBRARY_PATH);
function autoload($className) {
     $fileName = strtolower(str_replace('_', '/', $className));
     return spl_autoload($fileName);
spl_autoload_register('autoload');
This works perfectly on the dev machine, but on the production machine I get the error mentioned above.
Any ideas?
Thanks.

Answer 1

Thanks for the pointer. I removed the strtolower function since the original reason I put that there no longer exists.
The class names and file names all use the Zend naming convention where each word has its first letter capitalized. So, it is looking for the correct-casing of the file name.
It appears that case-sensativity isn't the issue I am dealing with here.
For now I just switched over to having the autoload function use require_once and gave it a file with a path. Not as good. But until I find the answer it will have to do.
Thanks again for trying to help. I appreciate it.