Better case is:
$this->assertSame(9199999999999999999, nextBigger(1999999999999999999));

i agree it will re execute the count in each iteration, but maybe he leave it like that to save 1 line of code at the cost of little eficiency.

A pleasure sir, it's always worth trying to formulate any constraints until you find a set that can be resolved.

Your testcase is wrong. The next bigger value of 1111111111111111119 is 1111111111111111191.

But your assumption is right: Searching for the solution iteratively is wasting lots of CPU, so this isn't qualifying as "clever" in the sense of "using an approach that is not brute-forcing it, but identifying the problem as what it truly is, and applying a very good solution to it".

But it might qualify for "clever" as in "applying a solution that isn't obvious".

I write that because the description asks for 10 decimals... see the examples too.
You passed because your first ten decimals are correct and because I don't test the number of decimals.

Yeah, good point.