I was reading that the interface methods must mention explicitly the parameters like this:
public function addInstrument(Instrument $instrument);

but in the book they just put this:
    public function addInstrument();

I'm new to interfaces, and I know a little about basic OOP. So, I don't want to be confused at this stage.

Indeed, in the downloaded ch02 code are more errors of the same kind as well as another syntax-error

    public function assignToBand(Band $band) {
 //error! $this->$bandReference = $band;
      $this->bandReference = $band;
    }

I'm working on that chapter after fixing those interfaces I got this in my browser:

Musician Jack Float

Catchable fatal error: Object of class Guitarist could not be converted to string in /var/www/php5_book/mycode/ch2/sampleCode.php on line 164