String similarity

PHP has a function that does it. How would I implement it? Considering string shifting and single character differences, that's a lot of cases to go through.


function similarity($a, $b) {
  $similarity = 0;
  for ($i=0; i < strlen($a); $i++) {
    if ($a[$i] == $b[$i])
      similarity++;
    else
      similarity--;
  }
  return $similiarity / strlen($a);
}