Substituting ph for f is common in some varieties of Internet slang. For example, it is common in 1337 or leetspeak, which is used by some subcultures on the Internet.
You can see their orthography in this neat chart:
Leet - Wikipedia, the free encyclopedia
So, sometimes, while this seems like a misspelling, when used by young people on the Internet or in other forms of electronic communication, it is actually spelled that way by design.
(This is not to say that some people don't simply misspell words.)
While some uptight people see this as the end of the universe and proof of the upcoming Armageddon, I see this as just another subculture argot which is actually pretty interesting. Also, the way leetspeak generates new words, especially different parts of speech, is fascinating.