multiple multipart uploads simultaneously amazon s3 php

Can I upload different files in different server using multipart uploads in Amazon s3 php at same time with same credentials?

I have created a application which will be in different servers when users click a button all the applications starts uploading files using same credentials.

try{
                     $s3 = S3Client::factory(array(
                   'key' => trim($as3_access_key),
                    'secret' => trim(str_replace(' ', '+', $as3_secure_key)),
                   // 'region' => 'us-west-2',
                    'region' => $as3_bucket_region,
                    'signature' => 'v4',
                //  'LocationConstraint' => 'eu-central-1',
                    'ssl.certificate_authority' => false
                ));



                $buckets = $s3->listBuckets();
            }
            catch (Exception $e){
                     return array(
                    'error' => 'Failed to upload to Amazon S3. Please check your details and set Managed Policies on your users to AmazonS3FullAccess.',
                    'error_code' => 'upload_failed_to_S3_check_your_details_and_set_managed_policies_amazonS3FullAccess_on_your_users',
                    'partial' => 1
                    );
            }


            if(filesize($backup_file) <5*1024*1024){


                try{
                   $s3->putObject(array(
                    'Bucket'     => $as3_bucket,
                    'SourceFile' => $backup_file,
                    'Key'        => $as3_file,
                    'ACL' => 'public-read'
                    ));


                   return true;

                }
                catch (Exception $e){
                    $err = $e->getMessage();
                    if($err){
                     return array(
                        'error' => 'Failed to upload to AmazonS3 ('.$err.').',
                        'error_code' => 'failed_upload_s3_with_error'
                    );
                 } else {
                    return array(
                        'error' => 'Failed to upload to Amazon S3.',
                        'error_code' => 'failed_upload_s3'
                    );
                 }
                }
            }
            else {


                    $filename = $backup_file;
                    $result = $s3->createMultipartUpload(array(
                        'Bucket'       => $as3_bucket,
                        'Key'          => $as3_file,
                        'ACL'          => 'public-read',

                    ));

                    $uploadId = $result['UploadId'];
                    echo $uploadId;
                    // 3. Upload the file in parts.
                    try {    
                        $file = fopen($filename, 'r');
                        $parts = array();
                        $partNumber = 1;
                        echo $partNumber;
                        while (!feof($file)) {
                            $result = $s3->uploadPart(array(
                                'Bucket'     => $as3_bucket,
                                'Key'        => $as3_file,
                                'UploadId'   => $uploadId,
                                'PartNumber' => $partNumber,
                                'Body'       => fread($file, 5 * 1024 * 1024),
                            ));
                            $parts[] = array(
                                'PartNumber' => $partNumber++,
                                'ETag'       => $result['ETag'],
                            );


     echo "Uploading part {$partNumber} of {$filename}.\n";
                        }
                        fclose($file);
                    } catch (S3Exception $e) {
                        $result = $s3->abortMultipartUpload(array(
                            'Bucket'   => $as3_bucket,
                            'Key'      => $as3_file,
                            'UploadId' => $uploadId
                        ));



                        echo "Upload of {$filename} failed.\n";
                    }

                    // 4. Complete multipart upload.
                    $result = $s3->completeMultipartUpload(array(
                        'Bucket'   => $as3_bucket,
                        'Key'      => $as3_file,
                        'UploadId' => $uploadId,
                        'Parts'    => $parts,
                    ));
                    $url = $result['Location'];


                    echo "Uploaded {$filename} to {$url}.\n";
                    return true;

            }

it works fine but sometimes i get error like following

PHP Fatal error occurred: Uncaught Aws\S3\Exception\MalformedXMLException: AWS Error Code: MalformedXML, Status Code: 400, AWS Request ID: A5F83D5285F85FCC, AWS Error Type: client, AWS Error Message: The XML you provided was not well-formed or did not validate against our published schema, User-Agent: aws-sdk-php2/2.7.27 Guzzle/3.9.3 curl/7.39.0 PHP/5.6.3 thrown in E:\xampp\htdocs\1\wp-content\plugins\iwp-client\lib\amazon\vendor\aws\aws-sdk-php\src\Aws\Common\Exception\NamespaceExceptionFactory.php on line 91.

is this function using same credentials,same bucket at simultaneously in different location.