Feed retrive API

feed.php

<?php
set_time_limit(0);
header('Content-Type: application/json');
header("Access-Control-Allow-Origin: *");
header("Access-Control-Allow-Headers: *");

define('INVALID', '<?xml version="1.0" encoding="UTF-8"?>
<rss xmlns:atom="http://www.w3.org/2005/Atom" xmlns:georss="http://www.georss.org/georss" xmlns:twitter="http://api.twitter.com" xmlns:dc="http://purl.org/dc/elements/1.1/" version="2.0">
  <channel>
  </channel>
</rss>');

function getRSS($url){
  $response = file_get_contents($url);
  if (($header = array_values(preg_grep($pattern, $http_response_header))) &&
    (preg_match($pattern, $header[0], $match) !== false))
  {
    $content_type = $match[1];
    if(strpos($content_type, 'xml') !== false){
        return $response;
    }
  }
  return INVALID;
}

$rss = [];
$feeds = [];
if(isset($_GET['twitter'])){
    $rss['twitter'] = getRSS('http://twitrss.me/twitter_user_to_rss/?user='.$_GET['twitter']);
}
if(isset($_GET['instagram'])){
    $rss['instagram'] = getRSS('http://rsshub.app/instagram/user/'.$_GET['instagram']);
}
if(isset($_GET['ncm'])){
    $rss['ncm'] = getRSS('http://rsshub.app/ncm/user/playlist/'.$_GET['ncm']);
}
if(isset($_GET['github'])){
    $rss['github'] = getRSS('http://feedmix.novaclic.com/atom2rss.php?source=https://github.com/'.$_GET['github'].'.atom');
}
if(isset($_GET['telegram'])){
    $rss['telegram'] = getRSS('http://rsshub.app/telegram/channel/'.$_GET['telegram']);
}

if(isset($_GET['dryrun'])){
    $dryrun = true;
};
foreach ($rss as $key => $xml) {
    try{
        $node = new SimpleXMLElement($xml);
        foreach($node->channel->item as $item){
            $counter ++;
            array_push($feeds, [
                'source'        => $key,
                'title'         => (string)$item->title,
                'description'   => (string)$item->description,
                'date'          => (string)$item->pubDate,
                'link'          => (string)$item->link
            ]);
        }
    } catch(Exception $ignored){}
}
function date_compare($a, $b)
{
    $t1 = strtotime($a['date']);
    $t2 = strtotime($b['date']);
    return $t2 - $t1;
}
usort($feeds, 'date_compare');
print(json_encode($feeds));