Add code syntax highlighting for PHP blocks in readme

Turns out to get syntax highlighting for PHP you have to use fenced code blocks (```php) rather than indented, AND you have to wrap it in <?php ?> tags, or else it won't get highlighted.

Bit of a pain having to wrap everything in <?php ?> tags, but I think the doco definitely looks better with syntax highlighting. Check out the readme of my repo to see how it looks, and take the pull request if you like it.