Thanks for sharing your notes with moving a plugin to the mu-plugins directory using MU Domain Mapping plugin. This domain_mapping_plugins_uri() function is a filter called by the plugins_url() function. A comment indicates this function “fixes the plugins_url”. However, it only appears to recreate the the same url produced in the plugins_url() function. I wonder if this was once a bug that has since been fixed in recent WordPress releases.

Have you tested just commenting out the line:

add_filter( ‘plugins_url’, ‘domain_mapping_plugins_uri’, 1 );

Also, one problem with domain_mapping_plugins_uri() is the use of get_option(‘site_url’) instead of site_url() which would return https if SSL is enabled.

You might want to consider the following:

// fixes the plugins_url
function domain_mapping_plugins_uri2( $full_url, $path=NULL, $plugin=NULL ) {
$plugins_dir = ( false !== strpos($full_url, ‘wp-content/mu-plugins’) ) ? ‘wp-content/mu-plugins’ : ‘wp-content/plugins’;
return site_url() . ‘/’ . strstr( $url, $plugins_dir );
}

I applied a few other optimizations to the code.