Whew vs phew

Writing the last post got me thinking, when would you use whew and when would use phew?
Yeah, my world is plagued by unanswered questions like these.Anyway, here is the answer
Phew- used to express relief or fatigue
Whew- a whistling sound or a sound like a half-formed whistle uttered as an exclamation
-used interjectionally chiefly to express amazement, discomfort, or relief

So u see, whew! can be a Phew! but Phew! cannot be a whew!.
as in
Whew! I am amazed at my findings,
Phew! I am glad this post is over.