This post will illustrate various handy file operations available in Drupal
Get Absolute Path of a file from URI : drupal_realpath()
A URI in drupal looks like: private://donor_registration/somefile.csv for a private file
Absolute Path: /srv/www/example.com/public_html/sites/default/private/files/donor_registration/somefile.csv
Get the filename URI path from the dir_name located under public files directory.
Save data in the destination. If file exists with the name, replace
Get the download link.
We have a site that we had been migrating since Drupal 4. Few days back when we tried to migrate the same from Drupal 6 to Drupal 7. There was an issue with respect to file location that was present since previous upgrades. To resolve that a symlink was created called "files" with linked to sites/default/files during D6 upgrade. This led to first loss of many files as Drupal does not handle symlink based file saving well. Secondly the files created were saved under public://files/filename.ext instead of public:://filename.ext which caused created files not being accessible.