Metaphone algorithms are designed to produce an approximate phonetic representation, in ASCII, of regular "dictionary" words and names in English and some Latin-based languages. It is intended for indexing words by their English pronunciation. It is one of the more popular of the phonetic algorithms and was published by Lawrence Philips in 1990. A Metaphone is up to ten characters in length.
It is used for fuzzy searches for records where each string to be searched has an index with a Metaphone key. You search for all records with the same or similar metaphone key and then refine the search by some ranking algorithm such as Damerau–Levenshtein distance. Metaphone searches are particularly popular with ‘ancestor’ sites that search on surnames where spellings vary considerably for the same surname. The current version, Metaphone 3, is actively maintained by Lawrence Philips, developed to account for all spelling variations commonly found in English words, first and last names found in the United States and Europe, and non-English words whose native pronunciations are familiar to English-speakers. The source of Metaphone 3 is proprietary, and Lawrence charges a fee to supply the source.
There is a version of Metaphone that is built into in PHP, and I have used the original public domain source of Metaphone, and modified it slightly to get the same result as the PHP version.
I have commented the source as clearly as I can to explain what is going on.