Doesn't return false if the key is not in the database (contrary to what the documentation states).
Returns -1 (TRUE) if the key doesn't exist
Returns 0 (FALSE) if the key does exist
This is still a bug (3.0.16 and 4.0.2) and it can still cost you your Saturday afternoon if you comply with the PHP documentation on TRUE (non-zero) and FALSE (0) with regards to this function.
manual/language.expressions.php