按日期排序多个XML文件 - Sort Multiple XML Files By Date

我有一个新闻小部件，它使用DOM Xml拉入多个Xml文件来显示。我正在设法弄清楚如何让文章按日期顺序显示。下面是小部件的代码:

$dh = opendir('./xml/');
    $fileCount = 0;
    while ($file = readdir($dh) and $fileCount < 3) {
    if (preg_match("/^..?$/", $file)) {
        continue;
    }
    $open = "./xml/".$file;
    $xml = domxml_open_file($open);
    //we need to pull out all the things from this file that we will need to 
    //build our links
    $root = $xml->root();
    $stat_array = $root->get_elements_by_tagname("status");
    $status = extractText($stat_array);
    $date_array = $root->get_elements_by_tagname("date");
    $date = extractText($date_array);
    $ab_array = $root->get_elements_by_tagname("abstract");
    $abstract = extractText($ab_array);
    $h_array = $root->get_elements_by_tagname("headline");
    $headline = extractText($h_array);
    $img_array = $root->get_elements_by_tagname("image");
    $image = extractText($img_array);
    $lead_array = $root->get_elements_by_tagname("para-intro");
    $para["intro"] = extractText($lead_array);
    if ($status != "live") {
        continue;
    }
    echo "<div class='"col-md-12 newsbox'"><img style='"margin-bottom: 10px;'" width='"100%'" src='"images/news/".$image."'"><div class='"newsboxtext'"><a href='"showArticle.php?file=".$file. "'"><h2 class='"mainheadline2'"> ".$headline . "</h2></a><a href='"showArticle.php?file=".$file . "'"><button  style='"margin-top:5px;'" type='"button'" class='"btn btn-sm btn-default'">+ Read More</button></a></div><hr class='"linedivider'">
</div>";
    $fileCount++;
}

这是我用来尝试排序结果的:

function date_compare($a, $b)
{
    $date1 = strtotime($a['date']);
    $date2 = strtotime($b['date']);
    return $date1 - $date2;
}    
usort($file, 'date_compare');

但是我收到了这个错误。警告:usort()期望参数1为array，

中给出null

考虑将所有提取的标签值传递到多维数组$xmlData中，然后对该数组进行排序，然后从排序后的数组中返回web html数据:

$r = 0;
$xml = new DOMDocument('1.0', 'UTF-8');
function extractText($tag) {
   foreach ($tag as $item) {
      $value = $item->nodeValue;
   }
   return $value;
}
while ($file = readdir($dh) and $fileCount < 3) {
    if (preg_match("/^..?$/", $file)) {
        continue;
    }
    $open = "./xml/".$file;
    $xml->load($open);
    //we need to pull out all the things from this file that we will need to 
    //build our links
    $xmldata = [];
    $xmldata[$r]['file'] = $file;
    $stat_array = $xml->getElementsByTagName("status");
    $xmldata[$r]['status'] = extractText($stat_array);
    $date_array = $xml->getElementsByTagName("date");
    $xmldata[$r]['date']= extractText($date_array);
    $ab_array = $xml->getElementsByTagName("abstract");
    $xmldata[$r]['abstract'] = extractText($ab_array);
    $h_array = $xml->getElementsByTagName("headline");
    $xmldata[$r]['headline'] = extractText($h_array);
    $img_array = $xml->getElementsByTagName("image");
    $xmldata[$r]['image'] = extractText($img_array);
    $lead_array = $xml->getElementsByTagName("para-intro");
    $xmldata[$r]['paraintro'] = extractText($lead_array);
    if ($status != "live") {
        continue;
    }
    $fileCount++;
    $r++;
}
// YYYY-MM-DD STRINGS STILL SORTS IN DAILY ORDER (I.E., '2015-01-01' < '2015-12-31')
usort($xmldata, function($a, $b) {
    return strcmp($a['date'], $b['date']);
});
$xmlcount = sizeof($xmldata[0]);
// ITERATE THROUGH LOOP EVERY NEEDED ITEM IN SORT ORDER OF ARRAY
for ($i = 0; $i <= ($xmlcount - 1); $i++) {
    echo "<div class='"col-md-12 newsbox'">
           <img style='"margin-bottom: 10px;'" width='"100%'" src='"images/news/".$xmldata[$i]['image']."'">
            <div class='"newsboxtext'">
              <a href='"showArticle.php?file=".$xmldata[$i]['file']. "'">
                <h2 class='"mainheadline2'"> ".$xmldata[$i]['headline']. "</h2>
              </a>
              <a href='"showArticle.php?file=".$xmldata[$i]['file']. "'">
                <button  style='"margin-top:5px;'" type='"button'" class='"btn btn-sm btn-default'">+ Read More</button>
              </a>
            </div><hr class='"linedivider'">
          </div>";
}