Proof of concept - Cache External WP Assets

asset-cache.php

<?php
/**
 * Proof of concept for https://github.com/divimode/gdpr-cache-script-styles
 */


/**
 * Filter all scripts and style URLs.
 * When an external URL is detected, download the file
 * and replace it in the page source.
 */
add_filter( 'scripts_loader_src', 'maybe_replace_url' );
add_filter( 'style_loader_src', 'maybe_replace_url' );

/**
 * Takes any URL as input, and always returns a local
 * URL.
 *
 * If an external URL is provided, the external asset
 * is downloaded to the uploads-folder and the function
 * return value is a URL to that local file.
 */
function maybe_replace_url( $url ) {
	if ( is_external_url( $url ) ) {
		if ( cache_asset( $url ) ) {
			$url = WP_CONTENT_URL .
				'/uploads/gdpr-cache/' .
				md5( $url );
		}
	}

	return $url;
}

/**
 * Returns true, if the specified URL points to an external asset.
 */
function is_external_url( string $url ) : bool {
	// Relative URLs are always local to the current domain.
	if ( 0 === strpos( $url, '/' ) ) {
		return false;
	}

	// Get protocol-relative
	$home_url = preg_replace( '/^\w+:/', '', home_url() );

	return false === strpos( $url, $home_url );
}

/**
 * Downloads the given URL to the websites uploads-folder.
 * Returns true on success.
 */
function cache_asset( $url ) {
	$cache_dir  = WP_CONTENT_DIR . '/uploads/gdpr-cache/';
	$cache_path = $cache_dir . md5( $url );

	$resp = wp_safe_remote_get(
		$url,
		[
			'timeout'  => 300,
			'stream'   => true,
			'filename' => $cache_path,
		]
	);

	if ( is_wp_error( $resp ) ) {
		@unlink( $cache_path );

		return false;
	}

	return true;
}