function bb2html($text)
{
  $bbcode = array("<", ">",
                "[list]", "[*]", "[/list]",
                "[img]", "[/img]",
                "[b]", "[/b]",
                "[u]", "[/u]",
                "[i]", "[/i]",
                '[color="', "[/color]",
                "[size="", "[/size]",
                '[url="', "[/url]",
                "[mail="", "[/mail]",
                "[code]", "[/code]",
                "[quote]", "[/quote]",
                '"]');
  $htmlcode = array("&lt;", "&gt;",
                "<ul>", "<li>", "</ul>",
                "<img src="", "">",
                "<b>", "</b>",
                "<u>", "</u>",
                "<i>", "</i>",
                "<span style="color:", "</span>",
                "<span style="font-size:", "</span>",
                '<a href="', "</a>",
                "<a href="mailto:", "</a>",
                "<code>", "</code>",
                "<table width=100% bgcolor=lightgray><tr><td bgcolor=white>", "</td></tr></table>",
                '">');
  $newtext = str_replace($bbcode, $htmlcode, $text);
  $newtext = nl2br($newtext);//second pass
  return $newtext;
}

Simply call the function by passing your string you wish to parse. To convert the parser to HTML to BB Code, change this line:

$newtext = str_replace($bbcode, $htmlcode, $text);
to
$newtext = str_replace($htmlcode, $bbcode, $text);

BB Code to HTML

PHP