Cache FeedBurner circulation (subscribers) results in PHP

functions.php

<?php
function get_feedburner_circulation($feedburner_id, $ttl = 86400) {
	$cache = dirname(__FILE__) . '/cache/feedburner.txt';

	// Check cache validity
	if ( !file_exists($cache) || ( filemtime($cache) + $ttl - time() < 0 ) ) {
		// try to get the feed data
		$curl = curl_init('https://feedburner.google.com/api/awareness/1.0/GetFeedData?uri=' . $feedburner_id);
		curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
		$result = curl_exec($curl);
		curl_close($curl);

		// unless valid query web service
		if ( $xml = @simplexml_load_string($result) ) {
			$circulation = $xml->feed->entry['circulation'];

			// and unless zero, cache result
			if ( $circulation > 0 ) file_put_contents($cache, "$circulation");
		}
	}

	// return cached result
	return file_get_contents($cache);
}
?>

Appreciate this code. This solves the 0 problem that everyone experiences. You should also build in comma delimiters like the prettynumbers() script - http://tympanus.net/codrops/2010/01/03/prettynumber-a-jquery-plugin-for-formatting-numbers/

Thank you for your comment, treycopeland. I believe it is better for the function to return unformatted output and the formatting to happen afterwards:

<?php echo number_format( get_feedburner_circulation($feedburner_id) ); ?>