The expression $stack of type array<integer,array<inte...\DecisionNode|integer>> is implicitly converted to a boolean; are you sure this is intended? If so, consider using ! empty($expr) instead to make it clear that you intend to check for an array without elements.
This check marks implicit conversions of arrays to boolean values in a comparison. While in PHP an
empty array is considered to be equal (but not identical) to false, this is not always apparent.
Consider making the comparison explicit by using empty(..) or !empty(...) instead.