Generate a histogram of HTML elements used on a web page

dom-histogram.php

<?php

class ElementIterator extends FilterIterator implements RecursiveIterator {
  public function __construct($nodelist) {
    return parent::__construct(new IteratorIterator($nodelist));
  }

  public function getChildren() {
    return new self($this->current()->childNodes);
  }

  public function hasChildren() {
    return !empty($this->current()->childNodes);
  }

  public function accept() {
    return isset($this->current()->tagName);
  }
}

function count_tags($url) {
  $doc = new DOMDocument;
  @$doc->loadHTMLFile($url);

  $it = new RecursiveIteratorIterator(
    new ElementIterator($doc->childNodes),
    RecursiveIteratorIterator::SELF_FIRST
  );
  $elems = iterator_to_array($it, FALSE);
  $tags = array_map(function($elem) { return $elem->tagName; }, $elems);
  $counts = array_count_values($tags);
  arsort($counts);
  return $counts;
}

$url = $argv[1];
$histogram = count_tags($url);
foreach ($histogram as $tag => $count) {
  printf("%-10s  %5d\n", $tag, $count);
}