Change dns-prefetch to preconnect with correct protocol

I have this function in my functions.php file:

function dns_prefetch_to_preconnect( $urls, $relation_type ) {
    if ( 'dns-prefetch' === $relation_type ) {
        $urls = [];
    }

    if ( 'preconnect' === $relation_type ) {
        $urls = wp_dependencies_unique_hosts();
    }

    return $urls;
}
add_filter( 'wp_resource_hints', 'dns_prefetch_to_preconnect', 0, 2 );

It takes the URLs defined in wp_dependencies_unique_hosts() – which WordPress by default assigns to the dns-prefetch link tag – and reassigns them to the preconnect link tag. The function was provided to me here:

Change dns-prefetch to preconnect for external enqueued resources

However, this function isn’t working entirely correctly. It adds the preconnect URLs using http instead of https.

Example: when I’m not using the above function, WordPress adds this link to my header:

<link rel="dns-prefetch" href="https://fonts.googleapis.com" />

And when I enable the above function, it replaces that link with this link:

<link rel="preconnect" href="http://fonts.googleapis.com" />

The problem, of course, is that it should be https, not http.

Can someone help me modify my function so that it gives me https links?

The problem is not that the function you’re using adds http:, the problem is it adds no URL schema at all!

As a result, WP needs to add a URL schema to turn the host into a URL, and so it uses http://. It has no way of knowing what the original was, or if the host supports HTTPS, so http:// is the safe bet.

If however you passed the array with URL schema added, it would be passed through without issue.

Something like this may do the trick:

$hosts = wp_dependencies_unique_hosts();
$urls = array_map( function( $host ) {
    return set_url_scheme( $host, 'https' );
}, $hosts );

In the longrun though, it would be better to get the actual URLs and distill the host URL out of them, rather than relying on wp_dependencies_unique_hosts if you wanted to preserve the mixture of http and https