Answered.

Your function should return false even if a numerical string representing an integer is provided
Failed asserting that 210 is false.

The test message already told you what is happening ;-)

I tested your code by adding echo($n); before $a = [];.
In its random test, for example n=261, the error message started by the following message.

Failed asserting that two arrays are equal.
Expected: Array (
    0 => '162'
    1 => '062'
    ...

By scrolling down, I also saw the following message that should be the expected data.

Actual  : Array (
    0 => 261
    1 => 260