Rss feed not displaying as feed in browsers - php

Question

<?php
include 'classes/db.connect.php';
if (isset($_GET['user'])) {
if($blog->blog_check($_GET['user'])) {
$username = strip_tags($_GET['user']);
} else {
http_response_code(404); include('html/blogs/404.html'); die;
}
} else {
http_response_code(404); include('html/blogs/404.html'); die;
}
$current_page = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off' ? 'https' : 'http') . '://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
$url = $blog->get_blogger_branding($username, 'gravurl');
$img = 'assets/blogs/images/'.$_GET['user'].'.png';
file_put_contents($img, file_get_contents($url));
$fbimg = 'https://afpayday.com/assets/blogs/images/'.$_GET['user'].'.png';
header('Content-Type: application/rss+xml');
echo '<?xml version="1.0" encoding="UTF-8" ?>
<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
<channel>
<title>'.ucfirst($blog->get_blogger_branding($username, "fname")).'\'s Payday Blog</title>
<description>Learn about affiliate marketing and affiliate product reviews.</description>
<link>https://afpayday.com/'.$username.'</link>
<copyright>Copyright 2020 Affiliate Payday</copyright>
<language>en-us</language>
<managingEditor>admin#afpayday.com (Bruce Bates)</managingEditor>
<pubDate>Sun, 23 Feb 2020 09:50:44 -0700</pubDate>
<webMaster>admin#afpayday.com (Bruce Bates)</webMaster>
<atom:link href="https://afpayday.com/'.$username.'/rss/" rel="self" type="application/rss+xml" />
<image>
<url>'.$fbimg.'</url>
<title>'.ucfirst($blog->get_blogger_branding($username, "fname")).'\'s Payday Blog</title>
<link>https://afpayday.com/'.$username.'</link>
<description>'.ucfirst($blog->get_blogger_branding($username, "fname")).'\'s Photo</description>
<width>144</width>
<height>144</height>
</image>
<item>
<title>Test</title>
<guid>https://afpayday.com/'.$username.'</guid>
</item>
</channel>
</rss>
';
?>
According to the w3 validator my feed is correctly coded. https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fafpayday.com%2Fviraladmin%2Frss%2F
However browsers are not treating it as I would expect.
Firefox tries to download the feed as a file.
Chrome, Opera, and Edge show the feed as code instead of displaying it as a document tree which is what I normally see when viewing an RSS feed.
Only Internet Explorer seems to be handling it properly and showing the "would you like to subscribe to this feed" option.
The feed is here: https://afpayday.com/viraladmin/rss/
I would expect it to look more like https://www.feedforall.com/sample.xml in a browser
What am I doing wrong?