Improving my PHP autoloader
What you've written is more or less the standard implementation of spl_autoload().

This is an equivalent approach:

$paths = array(
    get_include_path(),
    ROOT . DS . 'library'  . DS . 'intranet'  . DS  . 'classes',
    ROOT . DS . 'application' . DS . 'controller',
    ROOT . DS . 'application' . DS . 'model',
    ROOT . DS . 'application' . DS . 'view',
);

// help system to find your classes
set_include_path(join(PATH_SEPARATOR, $paths));

// use standard auto loader
spl_autoload_register();


Using namespaces in your application would help a lot as well; for instance, in each of your controller classes you simply add:

namespace controller;


When you take a similar approach for models and views as well, you reduce above code to this:

$paths = array(
    get_include_path(),
    ROOT . DS . 'library'  . DS . 'intranet'  . DS  . 'classes',
    ROOT . DS . 'application',
);

set_include_path(join(PATH_SEPARATOR, $paths));

// use standard auto loader
spl_autoload_register();

// ...
$x = new controller\something();


Namespaces are converted to directories by spl_autoload() so you only need to set the application directory to make it work.