PHP - use of bareword in interface function declarations

Question

Looking at PHP's documentation about interfaces, specifically here: PHP: Object Interfaces - Manual. The following code is given as a working example. Could someone explain what the bareword 'Baz' being declared as part of the function signature is please?

<?php
interface a
{
    public function foo();
}

interface b extends a
{
    public function baz(Baz $baz);
}

// This will work
class c implements b
{
    public function foo()
    {
    }

    public function baz(Baz $baz)
    {
    }
}

Answer 1

It is called type hinting.

The baz() method expects the first argument, $baz, to be an object of the type Baz. An object's type comes from either the class that it is built from, or from an interface that it implements.