This will cause an error in the case where invalid salts are provided for other algorithms. Currently, these invalid salts will silently fall back to STD_DES which is extremely weak. By detecting invalid DES salts, we can alert the user that there is a bug in their code.
The error is currently E_DEPRECATED as this has potential to break currently working (yet insecure) code. In the future it should be changed to an E_WARNING and return *0