Why is the "ph" pronounced like a "v" in "Stephen"? Is this the only word like that?

Stephen is of Greek origin (Stephanos). The ph is unusual in its v pronunciation. The only possibility I can think of is that it arrived in English via Spanish, where it is spelt 'Esteban'. The Spanish pronunciation of 'b' is somewhere between English 'V' and 'B'. In Hungarian it is spelt 'Istvan', so there's another possible source.

Is there any other instance of "ph" being pronounced as "v" in either American or British English?

In British English, nephew can be pronounced nev-yoo.

Born in London, I have always pronounced nephew with a 'v' sound. Locals in Essex (England) also pronounce the village name of Bulphan as 'Bulvan' rather than 'Bulfan', even though the second syllable is derived from the word fen.

I always found interesting that both "phial" and "vial" exist in English, having a common origin but different pronunciations. OED lists "vial" as an alteration of "phial"; the "ph" is originally Greek.

Does this count?

As one bearing the name of Stephen, I have read a little etymology and it seems that many people try to answer the question by pointing to the Greek Stephanos, which is incorrect because the question is asked of English, not of Greek. Was there not a Norman King of England, Stephen, whose court spoke French? Could there be a precedent in old Norman for the use of ph in the name? As well, the biblical saint, and first christian martyr, Saint Stephen, is ALWAYS pronounced the way King Stephen, and Stephen King, are pronounced. Stephen's Green in Dublin and almost all uses of Stephen in Ireland, follow the English standard, not the Scottish (v) usage (like Robert Louis Stevenson). So the pattern of Stephen is historically, biblically, and descriptively pronounced as Steven and not as Stefan, although I grew up with a Polish lad, Stephan, who prounced his name as Stefan. So it all depends on your preferred national standard. I'm sticking with King Stephen and Saint Stephen, thank you very much.

"STEVEN" is the canon pronunciation because Old English make-shift gender phonetics are applied to the E-PH. However in common rumor, a user of the name may be required to know the two pronunciations presented by text key models in probable instances rendering it differently pronounced than that of the "V" spelling. http://babynames.net/names/stephen