Tidy up PHP code examples in readme

Hi,

Just spotted this library after considering writing something similar myself - looks neat! I haven't had a chance to try it out yet, but I'm looking forward to it.

I just noticed that the PHP code examples in the readme could do with a little love, and since it was so quick to do I thought I'd submit a PR right here and now.

Changes are fairly obvious, but to be explicit, I have:
- Added PHP syntax highlighting
- Added missing 'function' keywords
- Referenced $family instead of the undefined variable $parent (which I assume was the original intent?)
- Made the spacing consistent (no newlines at the start of classes or functions)

Thanks!
Rowan