The Google Fonts Dropdown has built-in caching, which creates a cache file on the user’s machine so that you don’t have to call the API so many times.

There are three things that can be improved with this approach.

First is the direct use of the file system for caching, which will not work in any non-single-server environment. Caching the list using the Transients API is a much better and more efficient way.

Second is the use of fopen() and fwrite() to write to the file system, which should probably not work in a secure hosting environment for anything outside of wp-content/uploads. A good alternative is the proper use of the Filesystem API.

Third is the direct use of cURL, which means it will by-pass any logging or debugging scripts, proxy settings, timeouts, and will also fail if cURL happens to not be installed on the server. Using the WordPress HTTP API is the preferred way for remote requests.