Calculates the soundex key of
str.
Soundex keys have the property that words pronounced
similarly produce the same soundex key, and can thus be used
to simplify searches in databases where you know the
pronunciation but not the spelling. This soundex function
returns a string 4 characters long, starting with a
letter.
This particular soundex function is one described by
Donald Knuth in "The Art Of Computer Programming, vol. 3:
Sorting And Searching", Addison-Wesley (1973), pp.
391-392.
See also levenshtein(),
metaphone(), and
similar_text().