<?php
header('Content-type: text/xml');
$threshold = 600; // 10 minutes in seconds
$myApiUrl = 'http://myapisite.com/rss'; //rss feed

$file = 'filename.txt';
$fmtime = filemtime($file);
$timeDiff = time() - $fmtime;

/* gets the rss data from my api */
function get_data_from_api($url) {
	$ch = curl_init();
	$timeout = 20;
	curl_setopt($ch, CURLOPT_URL, $url);
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
	curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, $timeout);
	$data = curl_exec($ch);
	curl_close($ch);
	return $data;
}

// If file exists (fmtime is true) and timediff is lower or equal to threshold, send file.
if ($fmtime && $timeDiff <= $threshold) {
	$apiData = file_get_contents($file);
	echo $apiData;
// else if file doesn't exist or timediff is over threshold, fetch api and save to file.
} elseif (!$fmtime || $timeDiff > $threshold) {
	$apiData = get_data_from_api($myApiUrl);
	if($apiData !== false){
		file_put_contents($file, $apiData);
		echo $apiData;
	}
} else {
	// Do whaetever if nothing else happens
}

?>