For those without the BBCode extension, here's a relatively elegant function to do the trick.
Keep in mind that if you're using XHTML and one of your users tries to overlap lags <b>Like <i>so</b></i>, it will invalidate your markup. Still working on an expression for this.
<?php
function bb_parse($string) {
$tags = 'b|i|size|color|center|quote|url|img';
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': $replacement = "<strong>$innertext</strong>"; break;
case 'i': $replacement = "<em>$innertext</em>"; break;
case 'size': $replacement = "<span style=\"font-size: $param;\">$innertext</span>"; break;
case 'color': $replacement = "<span style=\"color: $param;\">$innertext</span>"; break;
case 'center': $replacement = "<div class=\"centered\">$innertext</div>"; break;
case 'quote': $replacement = "<blockquote>$innertext</blockquote>" . $param? "<cite>$param</cite>" : ''; break;
case 'url': $replacement = '<a href="' . ($param? $param : $innertext) . "\">$innertext</a>"; break;
case 'img':
list($width, $height) = preg_split('`[Xx]`', $param);
$replacement = "<img src=\"$innertext\" " . (is_numeric($width)? "width=\"$width\" " : '') . (is_numeric($height)? "height=\"$height\" " : '') . '/>';
break;
case 'video':
$videourl = parse_url($innertext);
parse_str($videourl['query'], $videoquery);
if (strpos($videourl['host'], 'youtube.com') !== FALSE) $replacement = '<embed src="http://www.youtube.com/v/' . $videoquery['v'] . '" type="application/x-shockwave-flash" width="425" height="344"></embed>';
if (strpos($videourl['host'], 'google.com') !== FALSE) $replacement = '<embed src="http://video.google.com/googleplayer.swf?docid=' . $videoquery['docid'] . '" width="400" height="326" type="application/x-shockwave-flash"></embed>';
break;
}
$string = str_replace($match, $replacement, $string);
}
return $string;
}
?>
[EDIT BY danbrown AT php DOT net: Contains a bugfix provided by (ramonvandam AT gmail DOT com) on 04-SEP-09 to address an improperly-defined parameter. Also contains a bugfix provided by (pompei2 AT gmail DOT com) on 15-FEB-10 to address improperly-closed tags. Plus, contains another bugfix provided by (angad AT wootify DOT com) on 18-JUL-2011 to fix an issue where unsupported tags provided to the function could cause the script to time out.]