How do i handle this dividing modulus error?

I have a function that generates a category menu. What i'm trying to do is to group two category in one ul. The problem is almos fixed but i'm getting an empty ul ( see the pic). How can I fix this ?

 function categorie($tab){
    global $mysql;
    $return = "";
    $categorie = $mysql->select(sprintf("SELECT * FROM categorii WHERE tab = '%d'", $tab));

    if($mysql->countRows() > 0){
        $i = 0;
            $return .= "<ul class=\"group\">\n";
                foreach ($categorie as $cat) {
                    if($i % 2 == 0){
                        $return .= "</ul><ul class=\"group\">";
                    }

                    $return .= "\t<li>\n";
                    $return .= "\t\t<ul>\n";
                    $return .= "\t\t\t<li class=\"head-list\">".$cat['categorie']."</li>\n";
                    $return .= $this->subcategorie($cat['categorie']);
                    $return .= "\t\t</ul>\n";
                    $return .= "\t</li>\n";
                    $i++;
                }
                $return .= "</ul>";

    }
    return $return;

}