why not support underscore?

I have a blocking doubt

why doesn't phalcon support underscore (_) separated class names or filenames while using namespaces. ?

I mean if we use $loader->registerNamespaces(); etc...

and what is alternative for that?

the alternative is to write your own loader which add PSR loading in phalcon

If i'm not wrong, Incubator has done this :

https://github.com/phalcon/incubator/blob/master/Library/Phalcon/Loader/PSR.php

Please, let me advice you to use composer instead. The autoloader is not an heavy feature of an application and composer is much more robust.