This is how I autoload all the classes in my controllers folder,
# auto load controller classes
function __autoload($class_name)
{
Everyone is is coping and pasting things from code they got off the internet (With the exception of the selected answer). They all use String Replace.
String Replace is 4 times slower than strtr. You should use it instead.
You should also use full paths when including classes with autoloading as it takes less time for the OS to resolve the path.