Uploading PHP Seamlessly

Hi, I can't find the answer to this question on google or these forums.

THIS IS NOT A QUESTION RELATED TO SMARTFTP DIRECTLY (I just have a question that hopefully someone can help me understand the answer to.)

How do large production website running on PHP upload their files?

I have a relatively large PHP file (~50K) that is accessed several times per second. Whenever I make a change and upload, the FTP server (proftp 1.3.2) deletes the current file and then uploads the new file. During the upload process, the file is unavailable and my site cannot function. This is not ideal; I would prefer that proftp uploads the file to a temporary file, and then replaces the file instantly. Is this an option/feature of proftp? I can do this manually, by changing the name first, uploading, and then using SSH I can overwrite the file with the replacement. But I wish the FTP software would do this for me.

Can anyone help me understand how large scale sites deal with this problem? Or if proftp can behave like this?

Please contact me directly through the support email.