Where does PHP save temporary files during uploading?

Its specified in upload_tmp_dir in your php.ini file. It is deleted by the system automatically after use.

php stores all temporary files, that includes uploaded files, in the temporary files directory as specified in the php.ini. Note that for uploads, those files might be removed as soon as the script the file was uploaded to was terminated (so unless you delay that script, you probably won't see the uploaded file). Another reason might be that the file is simply hidden on the file system.

So if you want to see the file, you might want to make sure you see all hidden files in the explorer and delay the script as long as you need to find the file.

It saves it at the path specified in $_FILES["file"]["tmp_name"], but deletes it after the script is done executing. It's up to you to move the file elsewhere if you want to preserve it.

You can check where php is currently saving your temp files $_FILES["file"]["tmp_name"] by printing

    sys_get_temp_dir()