tree menu php mysql list

I am trying to build an un-oredered list menu tree from my database in PHP and MySQL from here

I have an array of page objects I am returning from the db. Each page object has parent_id attribute, which is set to null if it doesn't have a parent. Here's what the page objects look like:

mydatabase

PHP Handling

$menu = mysqli_query($conn, "SELECT * FROM kategori ORDER BY id_kategori DESC");
    while($row=mysqli_fetch_assoc($menu)) {
        }   
function has_children($rows,$id) {
  foreach ($rows as $row) {
    if ($row['parent'] == $id)
      return true;
  }
  return false;
}
function build_menu($rows,$parent=0)
{  
  $result = "<ul>";
  foreach ($rows as $row)
  {
    if ($row['parent'] == $parent){
      $result.= "<li>{$row['nama_kategori']}";
      if (has_children($rows,$row['id_kategori']))
        $result.= build_menu($rows,$row['id_kategori']);
      $result.= "</li>";
    }
  }
  $result.= "</ul>";

  return $result;
}
echo build_menu($menu);

This isn't outputting the correct results for me, can anyone help me