function format(\DOMNode $node, $treeIndex = 0)
{
    //不格式化的标签
    if (in_array($node->nodeName, array("title", "p", "span")))
        return;
    if ($node->hasChildNodes()) {
        $treeIndex++;
        $tabStart = "\r\n" . str_repeat("TTT", $treeIndex);
        $tabEnd = "\r\n" . str_repeat("EEE", $treeIndex - 1);
        $i = 0;
        while ($childNode = $node->childNodes->item($i++)) {
            if ($childNode->nodeType == XML_TEXT_NODE) {
                if (preg_match('#^\s*$#', $childNode->nodeValue)) {
                    $node->removeChild($childNode);
                    $i--;
                    continue;
                }
                $childNode->nodeValue = trim($childNode->nodeValue);
            }
            $node->insertBefore($node->ownerDocument->createTextNode($tabStart), $childNode);
            $i++;
            $this->format($childNode, $treeIndex);
        };
        $node->appendChild($node->ownerDocument->createTextNode($tabEnd));
    }
}

$html = '
 
 
';
$doc = new \DOMDocument(); 
//$doc->formatOutput = true; //不知道是不是我的理解问题，这个选项格式化出来的并不完美
$doc->loadHTML($html); format($doc->documentElement);
echo $doc->saveHTML();