Pages system PHP/SQL

I'm working on a CMS, currently on being able to create static pages, manage menus etc. so.. I have This function that reads from the DB and finds all the static pages, and their position in the menu. I want to know what Do you think.

function update_menu(&$pdo, $filepath){
$qry = "SELECT * FROM pages WHERE top = 1";
$exec = $pdo->query($qry);
$string = "<ul>";
while($row=$exec->fetch(PDO::FETCH_ASSOC)){
    $string.= "<li><a href='?page=".$row['id']."'>".$row['title']."</a>";
        $qry = "SELECT * FROM pages WHERE parent = ?";
        $child_ex = $pdo->prepare($qry);
        $child_ex->bindValue(1, $row['id'], PDO::PARAM_INT);
        if($child_ex->execute()){
            $i=0;
            while($child=$child_ex->fetch(PDO::FETCH_ASSOC)){
                if($i==0){
                    $string.="<ul>";
                }
                $string.="<li><a href='?page=".$child['id']."'>".$child['title']."</a>";
                $i++;
            }
            if($i != 0)
                $string.="</ul>";
        }
    $string.="</li>";
}
$string.="</ul>";
file_put_contents($filepath, $string);

};

Output html:

<ul><li><a href='?page=1'>Page1</a><ul><li><a href='?page=4'>append1</a><li><a href='?page=5'>append2</a></ul></li><li><a href='?page=2'>page2</a><ul><li><a href='?page=6'>append1b</a></ul></li><li><a href='?page=3'>page3</a></li></ul>