The correct* spelling of "pho" requires the use of Vietnamese diacritical marks. If you have a Unicode-compliant application and want to impress your friends, you can spell "pho" correctly by using U+1EDF ("Latin small letter o with horn and hook above") for the last letter. If your browser fully supports the Unicode provisions of HTML 4.0 or newer and has the requisite fonts, this example (using the character reference "ở") will demonstrate the correct spelling: phở.
*Disclaimer: I do not know Vietnamese. The character I describe matches the one I have seen used most often in spelling "pho." Please feel free to /msg me with corrections.
Reference:
Unicode, Inc. "Latin Extended Additional." The Unicode Standard, Version 3.0. 2000. http://www.unicode.org/charts/PDF/U1E00.pdf (11 Mar. 2001).
Update (15 Mar 2001): I've noticed a fair number of Vietnamese restaurants spelling "pho" using an O that looks like the character I describe above, except the hook is pushed over to the right of the horn instead of being above the O. Unfortunately, I can't seem to find a single Unicode character that represents that, nor any combination of two characters (although the combination of U+01A1 followed by U+02C0 seems to come close).