Detecting whether a file is complete or partial in PHP

Simple

If you just want a simple technique that will handle most use cases - write a process to check the last modified date or (as indicated in the question) the file size and if it hasn't changed in a set time, say 1 minute, assume the upload is finished and process it. Infact - your question is a near duplicate of another question showing exactly this.

Robust

An alternative to polling the size/last modified time of a file (which with a slow or intermittent connection could cause an open upload to be considered complete too early) is to listen for when the file has finished being written with inotify.

Listening for IN_CLOSE_WRITE you can know when an ftp upload has finished updating a specific file.

I've never used it in php, but from the spartan docs it looks that that aught to work in exactly the same way as the underlying lib.

Make FTP clients work for you

Take into account how some ftp programs work, from this comment:

it's worth mentioning that some ftp servers create a hidden temporary file to receive data until the transfer is finished. I.e. .foo-ftpupload-2837948723 in the same upload path as the target file

If applicable, from the finished filename you can therefore tell if the ftp upload has actually finished or was stopped/disconnected mid-upload.

In theory, in case you are allowed to run system commands from PHP you can try to get the pids of your sftp server processes with pidof command and then use lsof output to check if open file descriptors exist for the currently checked upload file.

During a transfer, the only thing that knows the real size of the file is your SFTP client and maybe the SFTP server (don't know protocol specifics so the server may know too.) Unless you can talk to the SFTP server directly, then you'll have to wait for the file to finish. Checking at some interval seems kludgy but under the circumstances is pretty good. It's what we do on my project.

If you have a very short poll interval, it's not terribly resilient when dealing with uploads that may stall for a few seconds. So just set the interval to one a minute or something like that. Assuming you're on a *nix based platform, you could set up a cron job to run every minute (or whatever your polling interval is) to run your script for you.