int levenshtein (string str1, string str2)
This function return the Levenshtein-Distance between the two argument strings or -1, if one of the argument strings is longer than the limit of 255 characters.
The Levenshtein distance is defined as the minimal number of characters you have to replace, insert or delete to transform str1 into str2. The complexity of the algorithm is O(m*n), where n and m are the length of str1 and str2 (rather good when compared to similar_text(), which is O(max(n,m)**3), but still expensive).
See also soundex(), similar_text() and metaphone().