PHP DOM を使用して、特定のクラスを持つ HTML 要素からフラット配列にテキストを抽出する方法

<p class="Heading1-P">
    <span class="Heading1-H">Chapter 1</span>
</p>
<p class="Normal-P">
    <span class="Normal-H">This is chapter 1</span>
</p>

$heading = ['Chapter 1', 'Chapter 2', 'Chapter 3'];
$content = ['This is chapter 1', 'This is chapter 2', 'This is chapter 3'];

$dom = new DOMDocument();
$dom->loadHTML($test);

$xpath = new DOMXPath($dom);

$heading = parseToArray($xpath, 'Heading1-H');
$content = parseToArray($xpath, 'Normal-H');

query($xpathquery);

    $resultarray = [];
    foreach ($elements as $element) {
        $nodes = $element->childNodes;
        foreach ($nodes as $node) {
            $resultarray[] = $node->nodeValue;
        }
    }

    return $resultarray;
}

$test = <<
    Chapter 2


    This is chapter 2


    Chapter 3


    This is chapter 3

HTML;

$dom = new DOMDocument();
$dom->loadHTML($test);
$xpath = new DOMXPath($dom);
$heading = parseToArray($xpath, 'Heading1-H');
$content = parseToArray($xpath, 'Normal-H');

var_dump($heading);
echo "
";
var_dump($content);
echo "
";