PHP cURL chunked upload

Hi 

I have built a PHP to automate backups to dropbox amongst other things.

everything works well with the exception of chunked upload.

1. No Errors are returned from dropbox at any point in the transaction
2. I am using split to chunk the file into 50mb chunks
3. run a foreach loop to upload the chunks via 

 

 

"Dropbox-API-Arg: {\"cursor\": {\"session_id\": \"$session_id\",\"offset\": $offset},\"close\": false}";​

 

where $session_id is the session id & $offset = the amount of the file uploaded in bytes

 

• then finish up with 

 

 

"Dropbox-API-Arg: {\"cursor\": {\"session_id\": \"$session_id\",\"offset\": $offset},\"commit\": {\"path\": \"$upload_path\",\"mode\": \"overwrite\",\"autorename\": true,\"mute\": false}}";​

 

$offset = total file size

 

• All endpoints are correct.
• The problem is the file is uploaded to Dropbox with no errors, Dropbox reports the file size correctly, so far so good, then if this file is a tar and you download it & try and view the archive, it opens fine but only contains the directory listing and no files 
• if I reconnect all the chunks back together the archive reads fine so I can discount an error from the split utility
  any ideas ?