http://php.net/manual/en/class.datetimeinterface.php
DateTimeInterface is meant so that both DateTime and DateTimeImmutable can be type hinted for. It is not possible to implement this interface with userland classes.
| Version | Description |
|---|---|
| 5.5.8 | Trying to implement DateTimeInterface raises a fatal error now. Formerly implementing the interface didn't raise an error, but the behavior was erroneous. |