sync clock

[tomasnorre]

Resolves #845

[github-actions]

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[mk-mxp]

Also we should have a comment in the students stub that missing methods need to be added.

[mk-mxp]

This is one of the magic tricks in PHP, but we currently have no concept (concept exercise) for students to learn this. So we should either explain the magic in the stub (for now) or explicitly call the method as done before:

Suggested change

	$this->assertEquals('08:00', $clock);
	$this->assertEquals('08:00', $clock->__toString());

or

Suggested change

	$this->assertEquals('08:00', $clock);
	$this->assertEquals('08:00', (string)$clock);
	// plus use `implements Stringable` in the students stub
	// plus a comment in the stub pointing to PHP docs on interface `Stringable`

@homersimpsons What do you think about this?

[homersimpsons]

If we want to show magic methods with this exercise, I think I would prefer the explicit (string) cast, it has the advantages to clearly show that this method is magic.

But I do not know if we want to show those here.

I believe we could have a concept to show __toString, __set, __get which I think are rather "common". __construct is a magic method but I believe it is already being taught.

[mk-mxp]

These changes are not connected to the discussion about the magic method usage.

Please add a comment to the students stub, that all missing methods need to be added by the student.