function get_average_readers($feed_id,$interval = 7){
	$today = date('Y-m-d', strtotime("now"));
	$ago = date('Y-m-d', strtotime("-".$interval." days"));
	$feed_url="https://feedburner.google.com/api/awareness/1.0/GetFeedData?uri=".$feed_id."&dates=".$ago.",".$today;
	$ch = curl_init();
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
	curl_setopt($ch, CURLOPT_URL, $feed_url);
	$data = curl_exec($ch);
	curl_close($ch);
	$xml = new SimpleXMLElement($data);
	$fb = $xml->feed->entry['circulation'];

	$nb = 0;
	foreach($xml->feed->children() as $circ){
		$nb += $circ['circulation'];
	}

	return round($nb/$interval);
}

http://www.catswhocode.com/blog/10-life-saving-php-snippets

Get your average Feedburner subscribers

PHP