The words sonny, sunny sound the same but have different meanings and spellings. Why do sonny, sunny sound the same even though they are completely different words?
The answer is simple: sonny, sunny are homophones of the English language.
Used as a familiar form of address for a boy or young man.
Exposed to or abounding in sunshine: a sunny room.
Cheerful; genial: a sunny smile.
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.