Complete as many of the challenges as you can and let us know how you got on. Ask for hints if you get a bit stuck. Let us know if we made any mistakes or if you can come up with some better questions.
http://3v4l.org/ is useful if you need quick access to PHP.
Note: if you've solved some of these, don't publish them as a fork of this repo, or it will spoil the fun for everyone else :-). You can create a secret gist with a free Github account.
Have fun!
PS. Get in touch if you think you did OK - we are often hiring for PHP developers.