I have a seven year old cat and she is starting to vomit a foamy white (what is I think is phlem), what does that mean?

Are there any other symptoms? If not, then I'd not worry about it too much. Sounds like she just ate something that does not agree with her.

If she exhibits other bad symptoms, get her to a vet.

I don't normally venture into the cat section but if this were a dog I wouldn't be worried.
My dogs throw up foam every so often when their stomachs are a little upset.

If it is happening frequently or happens several times over a few days then I would book in to the vet.

When an animal is vomiting or has diarrhea the most important thing to do is increase their water intake. If she will drink pedialyte that is a great way to keep them hydrated.