If you’re upgrading from the v2 to v3 SDK or following the low level API documentation for Multipart uploads such as http://docs.aws.amazon.com/AmazonS3/latest/dev/LLuploadFilePHP.html you will run into an impassable error. I ran into this problem while working on adding the new S3 SDK into BackupBuddy.

The Error:

Error executing “CompleteMultipartUpload” on “https://s3.amazonaws.com/[…]”; AWS HTTP error: Client error: `POST https://s3.amazonaws.com/[…]` resulted in a `400 Bad Request` response:
InvalidRequestYou must specify at least one part

The problem is that the CompleteMultipartUpload function parameters have changed but Amazon has not updated their documentation properly and it is missing from their migration guide.

Solution:

The “Parts” parameter now needs to be placed in a new array named “MultipartUpload”.

Incorrect parameters (as per documentation or v2 SDK):
$result = $s3->completeMultipartUpload(array(
'Bucket' => $bucket,
'Key' => $key,
'UploadId' => $uploadId,
'Parts' => $parts,
));

Correct parameters for v3 SDK:
$result = $s3->completeMultipartUpload(array(
'Bucket' => $bucket,
'Key' => $key,
'UploadId' => $uploadId,
'MultipartUpload' => array(
'Parts' => $parts,
),
));

I’ve posted a post in the S3 forums ( https://forums.aws.amazon.com/thread.jspa?messageID=740339&#740339 ) requesting Amazon update their documentation so this may be fixed there at some point. Special thanks to this StackOverflow post: http://stackoverflow.com/questions/30383982/aws-s3-completemultipartupload-error