Why do I see underscore prefixed in front of functions and variables in PHP?

naming-conventions, php

Solution

It is a convention that means that the method/property is private

PS: as long as it is just a convention it could be applied to protected and anything else as well

EDIT 5/2016

This is now not recommended, see this

Problem

Is this a convention of some sort? I'm looking through some code, and at first I thought it might have been for private scopes, but that doesn't seem to be the case. I know double underscore is for PHP's magic methods, but I am not sure why the author of this Recurly library (https://github.com/recurly/recurly-client-php) is using underscores in front of functions and variables. Could someone please explain this to me?

Original source