Ensure data is sent to client?

Greetings, How would one go about knowing if the complete contents of a file was successfully sent to a client browser? In an application I am writing I have to restrict users to only downloading a file once, but if the download fails they should be able to have another attempt. I was looking and fpassthru and readfile, but neither of them seems to wait and see if the data was actually sent successfully, they just seem to dump to stdout and return. Is there any way to detect that the entire file contents actually made it to the client? Thanks, Matthew