@yayuj There's no magic in there. He got 4 classes, one of which is JsobSerializable with literally one method, that is covered in the MoneyTest. Other 3 classes have their tests MoneyTest, CurrencyTest and IntlFormatterTest.
There are also currencies that are simple implementations of Money, exceptions that are not tested of course and interfaces, again, not tested.
But speaking of - it is really good example of how to get 100% of real coverage, ie. by using @cover and @uses annotations.
First tells PHPUnit to include specified method in the coverage, latter tells the opposite - that the method is used in the flow but shouldn't be included in the coverage report.