Calculates the metaphone key of
Similar to soundex() metaphone creates the same key for similar sounding words. It's more accurate than soundex() as it knows the basic rules of English pronunciation. The metaphone generated keys are of variable length.
Metaphone was developed by Lawrence Philips <lphilips at verity dot com>. It is described in ["Practical Algorithms for Programmers", Binstock & Rex, Addison Wesley, 1995].