This is not the best practice, because it always runs O(n). Even if the first two chars in the string are equal, it would iterate on all other chars.

See https://phpunit.de/manual/current/en/appendixes.assertions.html#appendixes.assertions.assertEquals

Its easy to check:

function skiponacci($n) {
  return 'my actual result';
}

Outputs:

Failed asserting that two strings are equal.
Expected: 'my actual result'
Actual  : '1'

Nice kata!

In "example test cases" in assertEquals $result and expected value should be swapped.