Answered.

Ok, got that, but this error is super undescriptive...

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 ;-)

That is the error " Failed asserting that two arrays are equal." in the random task but how to solve it?
EDIT: Ok, got it it should be an array, not a string

I know what the error means lmao. Also, like I said, the directions and deliverables were not specified, next time maybe just try a bit harder when putting the kata together.

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