PHP BBCode Parser w/ line break support

bbparser.php

<?php
function bb_parse($string) {
  $string = str_replace("\r\n", "\r", $string);
  $tags = 'b|i|s|u|rain';
  while (preg_match_all('`\[('.$tags.')=?(.*?)\](.+?)\[/\1\]`', $string, $matches)) foreach ($matches[0] as $key => $match) {
    list($tag, $param, $innertext) = array($matches[1][$key], $matches[2][$key], $matches[3][$key]);
    switch ($tag) {
      case 'b': $replacement = "<b>$innertext</b>"; break;
      case 'i': $replacement = "<i>$innertext</i>"; break;
      case 's': $replacement = "<s>$innertext</s>"; break;
      case 'u': $replacement = "<u>$innertext</u>"; break;
      case 'rain': $replacement = "<span style=\"background-image: -webkit-gradient( linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2),color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22) );
background-image: gradient( linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2),color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22) );color:transparent;-webkit-background-clip: text;background-clip: text;\">$innertext</span>"; break;
    }
    $string = str_replace($match, $replacement, $string);
  }
  return $string;
}