spl_autoload_register() function has been around since late 2005, yet documentation on it is still pretty sparse.
spl_autoload_register() does basically the same thing as __autoload(), but – instead of being a callback defined in the global namespace (reminds me of ON ERROR GOTO in BASIC) – it allows you to add your autoload handler onto the end of a chain of callbacks. Useful if you don’t want to clobber everyone else’s autoload code.
More info here.