Так как <li> внутри тега <ul>
function bbcode($text){
$text = strip_tags($text);
$search = array(
'/\[b\](.*)\[\/b\]/is',
'/\[img\](.*)\[\/img\]/is',
'/\[url=(.*?)\](.*)\[\/url\]/is',
'/\[ul\](.*)\[\/ul\]/is',
'/\[li\](.*)\[\/li\]/is',
);
$replace = array(
"<b>$1</b>",
"<img src='$1'></img>",
"<a href='$1'>$2</a>",
"<ul>$1</ul>",
"<li>$1</li>",
);
$text = preg_replace($search,$replace,$text);
return $text;
}