BB code is a very simple set of instructions (also known as BB tags) that provide rules as to how a piece of text should be formatted. When a page that contains BB code is displayed in the browser, the BB tags are replaced with appropriate HTML tags that the browser can understand.
<?php
function BBCode ($string) {
$search = array(
\'\[b\](.*?)\[\/b\]\\',
\'\[i\](.*?)\[\/i\]\\',
\'\[u\](.*?)\[\/u\]\\',
\'\[img\](.*?)\[\/img\]\\',
\'\[url\=(.*?)\](.*?)\[\/url\]\\',
\'\[code\](.*?)\[\/code\]\\'
);
$replace = array(
\'<b>\\1</b>\',
\'<i>\\1</i>\',
\'<u>\\1</u>\',
\'<img src="\\1">\',
\'<a href="\\1">\\2</a>\',
\'<code>\\1</code>\'
);
return preg_replace($search, $replace, $string);
}
?>
<?php
echo BBCode(\'Here is some [b]Bold![/b] and so on!\');
?>
Source
http://www.sitepoint.com/bb-code-php-application/
http://www.pixel2life.com/forums/index.php?/topic/10659-php-bbcode-parser/