Miscellany

  function replace_at_pos(&$text, $pos, $previous, $with, $addpos = 0) {
    if (!is_int($previous))
      $previous = strlen($previous);
    $previous += $addpos;
    $text = substr($text, 0, $pos) . $with . substr($text, $pos+$previous);
    return strlen($with) - $previous;
  }