The words filter, philter sound the same but have different meanings and spellings. Why do filter, philter sound the same even though they are completely different words?
The answer is simple: filter, philter are homophones of the English language.
A porous material through which a liquid or gas is passed in order to separate the fluid from suspended particulate matter.
A device containing such a material, especially one used to extract impurities from air or water.
Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiations of certain frequencies while allowing others to pass.
A colored glass or other transparent material used to select the wavelengths of light allowed to reach a photosensitive material.
A love potion.
A magic potion or charm.
To enchant with or as if with a philter.
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.