function bbcode($input){

        $code = array('/\[b\](.*?)\[\/b\]/is',
                       '/\[i\](.*?)\[\/i\]/is',
                       '/\[u\](.*?)\[\/u\]/is',
                       '/\[url=(.*?)\](.+?)\[\/url\]/is',
                       '/\[quote\](.*?)\[\/quote\]/is',
                       '/\[color=(.*?)\](.*?)\[\/color\]/is');

        $replace = array('<strong>$1</strong>',
                      '<i>$1</i>',
                      '<u>$1</u>',
                      '<a href="$1">$2</a>',
                      '<br /><div class="quote">$1</div><br />',
                      '<span style="color: $1;">$2</span>');

        $text = preg_replace($code, $replace, $input);

     return $text;

}
echo bbcode('This is a [b]test[\b].');