The words fizz, phiz sound the same but have different meanings and spellings. Why do fizz, phiz sound the same even though they are completely different words?
The answer is simple: fizz, phiz are homophones of the English language.
To make a hissing or bubbling sound; effervesce.
A hissing or bubbling sound.
An effervescent beverage.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.