A sandbox is somewhere were you can play. That's what a PHP sandbox is, as well, a place where you can put in some PHP code snippets and see if they are going to work. This is a great place for you to experiment with commands to see how they actually work. While you can find many of them using a Search Engine, you can also follow this link to find one here at PHP Sandbox.
As an example follow that link and replace the code with the following:
Execute the code. What do you think each of the different parts of this code snippet do? Try deleting or changing things to get an idea of how this works together.