Standard Deviation of HTML Tag Depth

stddev.php

<?php
	function _getDomDepth(DomNode $node) 
	{ 
		$r = -2; 
		while ($node)
		{ 
			$r++;  
			$node = $node->parentNode; 
		} 
		return  $r; 
	}
	
	$page = file_get_contents('http://apply.embed.ly/static/data/2.html');
	
	libxml_use_internal_errors(true); 
	$document = new DOMDocument();
	$document->loadHTML($page);
	$dom = new DOMXPath($document);
		
	$xmlNodes = $dom->query('//*');
	
	$values = array();
	$second_values = array();
	$total = 0;
	$second_total = 0;
	
	foreach ($xmlNodes as $node)
	{
		if ($node->tagName == 'p')
			$values[] = _getDomDepth($node);
	}
	
	foreach ($values as $v)
		$total += $v;
	
	$average = $total / count($values);
	
	foreach ($values as $v)
		$second_values[] = ($v - $average)*($v - $average);
		
	foreach ($second_values as $sv)
		$second_total += $sv;
		
	$second_average = $second_total / count($second_values);
	
	$stddev = sqrt($second_average);
	
	print_r($stddev);
?>