PHP

TypeError: Return value of is_prime() must be of the type boolean, none returned

Problem is in solution returned just boolean (true or false), neither NULL, 1, 0.

My solution (PHP) pass sample tests in my computer, but don't pass in codewars. I wrote several solutions, they all works in my IDE.
Can sombody explane my mistake?

Failed test codewars:

Failed asserting that two arrays are equal.
Expected: Array (
0 => 'arp'
1 => 'live'
2 => 'strong'
)
Actual : Array (
0 => 'live'
1 => 'strong'
2 => 'arp'
)

Eqial function in my IDE:

Array
(
[0] => arp
[1] => live
[2] => strong
)

Nice kata. I started from this one: https://www.codewars.com/kata/alphabet-war/php and after wrote the solution for 'Airstrike'-version. But, unfortunately, there is no support PHP ((

Thanks =)