Formal:
* Magandang gabi po. (This is the most formal way to say good evening, using the respectful "po".)
Informal:
* Magandang gabi.
* G'night. (This is a common slang expression, similar to "good night" in English.)
Other options:
* Gabi na po. (This literally means "It's evening already" and is used in a casual setting.)
The best way to choose the appropriate greeting depends on the person you're talking to and the situation.