I'm not sure if this is a good idea, having a class behaving different without being able to check what method is uses.

also I think this should be it's own class, maybe extend Registry from a base class which doesn't implement deep merge. Also it could be interesting to convert between both methods...

But as this pr is I don't think it's a good approach tbh.

having a class behaving different without being able to check what method is uses.

Can add method to check it, if realy need.

this should be it's own class, maybe extend Registry from a base class which doesn't implement deep merge.

Also as an option, I can make as own class.
This basiclay write another Registry class, because almost all methods in Registry doing "deep check".

Any other opinions? :)

Maybe as alternative way, can do check for non empty separator before each call of:

strpos($path, $separator);
explode($separator, $path); 

This will be kind of the same (but not really).

Currently "empty separator" will cause error.

I made PR for empty separator #66

The pr for a new class #67 for flat registry

Looks good but I noticed that $data is a stdClass no idea why. Also I would make this class final.