The words phenyl, fennel sound the same but have different meanings and spellings. Why do phenyl, fennel sound the same even though they are completely different words?
The answer is simple: phenyl, fennel are homophones of the English language.
A Eurasian plant (Foeniculum vulgare) having pinnate leaves, clusters of small yellow flowers grouped in umbels, and aromatic seeds used as flavoring.
The edible seeds or stalks of this plant.
The univalent organic radical C6H5, derived from benzene by removal of one hydrogen atom.
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").