MU Plugin: Remote Images

remote-images.php

<?php

/**
 * Plugin Name: Remote Images
 */
 
if(!in_array($_SERVER['SERVER_NAME'], ['EXAMPLELOCALDOMAIN.test', 'ANOTHEREXAMPLE.test'])) {
    return;
}

// Replace src paths
add_filter('wp_get_attachment_url', function ($url)
{
    if(file_exists($url))
    {
        return $url;
    }
    return str_replace(WP_HOME, 'https://www.some-production-site.com', $url);
});

// Replace srcset paths
add_filter('wp_calculate_image_srcset', function($sources)
{
    foreach($sources as &$source)
    {
        if(!file_exists($source['url']))
        {
            $source['url'] = str_replace(WP_HOME, 'https://www.some-production-site.com', $source['url']);
        }
    }
    return $sources;
});