PHP throwing error on XML opening tag

I have a file that is including a file named sitemap.html and in the sitemap, I have the following code:

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">
    <url>
        <loc><?php echo SITE_URL . '/'; ?></loc>
        <changefreq>monthly</changefreq>
        <priority>1.00</priority>
    </url>

    <?php foreach ($this->data->content as $content): ?>
        <url>
            <loc><?php echo SITE_URL . $content->permalink; ?></loc>
            <lastmod><?php echo date("Y-m-d", $content->publish_date); ?></lastmod>
            <changefreq>monthly</changefreq>
            <priority><?php echo $content->sitemap_index; ?></priority>
        </url>
    <?php endforeach; ?>
</urlset>

Everything seems okay, but I'm getting an error 500 that's saying:

PHP Parse error: syntax error, unexpected 'version' (T_STRING) in sitemap.html on line 1

As you can see, I'm using <?xml and not <?php so why is it trying to parse it as PHP?

At first, I thought it was the magic quotes that are causing an issue, but when I do a var_dump of get_magic_quotes_gpc(), I get bool(false) so the magic quotes aren't even on.

Also, in my php.ini, I see that magic_quotes is OFF.

As an alternative to fix it, I replaced my first line with the following:

<?php echo '<?xml version="1.0" encoding="UTF-8"?>'; ?>

But I'm still curious about what's happening.

I have other websites hosted on the same server (PHP version 5.4.45), and I don't get the PHP Parse Error on the sitemap of the other websites...

Any idea what might be causing this error?

You should check short_open_tag option. The way i see it, PHP sees <? part of your <?xml as a php open tag and generates an error.