You're right! I came across this issue when I tried to use my isPrime implementation in a Kata.

The following test will fail:
Assert::That(false, Equals(PrimeNumber::isPrime(4)));

You should replace:
if (num % 2 == 0) return true;
with:
if (num == 2) return true;
if (num % 2 == 0) return false;