Formal:
* Magandang gabi po. (This is the most common and polite way to say good evening.)
Informal:
* Magandang gabi. (This is a more casual way to say good evening.)
* Gabi na. (Literally means "It's evening" and is a more casual way to acknowledge the time of day.)
Other options:
* Mabuti pang gabi. (This is a more literary way to say good evening.)
* Magandang hapon. (This is literally "good afternoon", but it can be used in the evening, especially if it's still light outside.)
The best way to choose which phrase to use is to consider the context and your relationship with the person you're speaking to.