取数：

$data = array();
while ($row = mysql_fetch_assoc($result)) {
  $itemName = $row["item_name"];
  if (!array_key_exists($itemName, $data)) {
    $data[$itemName] = array();
  }
  $data[$itemName][] = $row;
}

显示数据

foreach ($data as $itemName => $rows) {
  echo '<h1>', $itemName, '</h1>';
  echo '<ul>';
  foreach ($rows as $row) {
    echo '<li>', $row["article_name"], '</li>';
  }
  echo '</ul>';
}