Calculates the soundex key of string.
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 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.
The input string.
Returns the soundex key as a string with four characters. If at least one letter is contained in string, the returned string starts with a letter. Otherwise "0000" is returned.
|8.0.0||Prior to this version, calling the function with an empty string returned false for no particular reason.|