(PECL bbcode >= 0.9.0)
bbcode_create — Create a BBCode Resource
This function returns a new BBCode Resource used to parse BBCode strings.
An associative array containing the tag names as keys and parameters required to correctly parse BBCode as their value. The following key/value pairs are supported:
Returns a BBCode_Container
Beispiel #1 bbcode_create() example
<?php
$arrayBBCode=array(
    ''=>         array('type'=>BBCODE_TYPE_ROOT,  'childs'=>'!i'),
    'i'=>        array('type'=>BBCODE_TYPE_NOARG, 'open_tag'=>'<i>',
                    'close_tag'=>'</i>', 'childs'=>'b'),
    'url'=>      array('type'=>BBCODE_TYPE_OPTARG,
                    'open_tag'=>'<a href="{PARAM}">', 'close_tag'=>'</a>',
                    'default_arg'=>'{CONTENT}',
                    'childs'=>'b,i'),
    'img'=>      array('type'=>BBCODE_TYPE_NOARG,
                    'open_tag'=>'<img src="', 'close_tag'=>'" />',
                    'childs'=>''),
    'b'=>        array('type'=>BBCODE_TYPE_NOARG, 'open_tag'=>'<b>',
                    'close_tag'=>'</b>'),
);
$text=<<<EOF
[b]Bold Text[/b]
[i]Italic Text[/i]
[url]http://www.php.net/[/url]
[url=http://pecl.php.net/][b]Content Text[/b][/url]
[img]http://static.php.net/www.php.net/images/php.gif[/img]
[url=http://www.php.net/]
[img]http://static.php.net/www.php.net/images/php.gif[/img]
[/url]
EOF;
$BBHandler=bbcode_create($arrayBBCode);
echo bbcode_parse($BBHandler,$text);
?>
Das oben gezeigte Beispiel erzeugt folgende Ausgabe:
<b>Bold Text</b> [i]Italic Text[/i] <a href="http://www.php.net/">http://www.php.net/</a> <a href="http://pecl.php.net/"><b>Content Text</b></a> <img src="http://static.php.net/www.php.net/images/php.gif" /> <a href="http://www.php.net/"> [img]http://static.php.net/www.php.net/images/php.gif[/img] </a>
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) { 
        while (preg_match_all('`\[(.+?)=?(.*?)\](.+?)\[/\1\]`', $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.]