$msg = preg_replace(\"#\\[b\\](.*)\\[\\/b\\]#isU\", \'<b>\\\\1</b>\', $msg);
$msg = preg_replace(\"#\\[i\\](.*)\\[\\/i\\]#isU\", \'<i>\\\\1</i>\', $msg);
$msg = preg_replace(\"#\\[u\\](.*)\\[\\/u\\]#isU\", \'<u>\\\\1</u>\', $msg);
$msg = preg_replace(\"#\\[img\\](.*)\\[\\/img\\]#isU\", \'<img src=\"\\\\1\" width=\"100\" height=\"100\">\', $msg);
$msg = preg_replace(\"/\\[url=([^\\]]+?)\\](.+?)\\[\\/url\\]/uis\", \"<a href=\\\"$1\\\" rel=\\\"nofollow\\\">$2</a>\", $msg);
$msg = preg_replace(\"/\\[color=(.*)\\](.+?)\\[\\/color\\]/uis\", \'<span style=\"color:$1\">$2</span>\', $msg);

Свое ставь