Fix: Use class keyword

[localheinz]

This PR

• makes use of the class keyword instead of specifying class names by string literals

[yguedidi]

Thank you so much @localheinz!! How did you do that? Is there a tool that do that automatically? :)
@SammyK what do you think about this one?

[localheinz]

@yguedidi

I made use of @kalessil's PhpStorm plugin (see https://github.com/kalessil/phpinspectionsea) and then sorted out a few imports!

[SammyK]

I'm a huge fan of this since it makes refactoring a breeze with a good IDE. I'm glad the SDK can finally take advantage of this feature since the min PHP version will be 5.6. :) Thanks @localheinz!