The words fat, phat sound the same but have different meanings and spellings. Why do fat, phat sound the same even though they are completely different words?
The answer is simple: fat, phat are homophones of the English language.
The ester of glycerol and one, two, or three fatty acids.
Any of various soft, solid, or semisolid organic compounds constituting the esters of glycerol and fatty acids and their associated organic groups.
A mixture of such compounds occurring widely in organic tissue, especially in the adipose tissue of animals and in the seeds, nuts, and fruits of plants.
Animal tissue containing such substances.
Slang Excellent; first-rate: phat fashion; a phat rapper.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").