Interestingly there's no test for this. My code was essentially your code without rev() and it still passed.

Oh, I see, because of static $n.Thank you!

Print the evolution of $n and you will understand what happens from one test to the following. Hope that helps. They can't be successfull on your computer if one test is immediately following the previous one.

class PopulationGrowthCases extends TestCase
{
    private function revTest($actual, $expected) {
        $this->assertEquals($expected, $actual);
    }
    public function testBasics() {
        $this->revTest(nbYear(1500, 5, 100, 5000), 15);
        $this->revTest(nbYear(1500000, 2.5, 10000, 2000000), 10);
    }
}

print:

✘ Failed asserting that 25 matches expected 10.

Which test is failing?