Working with File Content Sent From API file download curl request (php code)

In this code, you're telling curl (via the 'CURLOPT_FILE' option) to save the downloaded data to the "$fp" file pointer that you provided. That means that the file data should be getting saved to the location specified by "filename" on the local filesystem. 

So, if you're building a web app where this code is running on your web server, the file is going to be downloaded to the web server's filesystem, and not automatically returned to the end-user's web browser.

You can serve the downloaded file or stream the data to the user's browser, but exactly how you do so will depend on your setup and web framework/library that you're using (if any). As that's not about using the Dropbox API though, I can't offer much help or specific details on how to do so. You may want to refer to the documentation for your programming language, web framework, and/or web server, etc. for information on how to do so.