On the AWS PHP SDK blog there's a new tutorial posted showing you how to use the multipart uploader with client-side encryption via the library.
The AWS SDK for PHP released support for multipart uploads with client-side encryption in version 3.48.0 via the S3EncryptionMultipartUploader. With client-side encryption, data is encrypted and decrypted directly in your environment. This means that this data is encrypted before it’s transferred to Amazon S3, and you don’t rely on an external service to handle encryption for you.
Multipart uploads are designed to improve the upload experience for larger objects. With it, you can upload objects via parts that can be uploaded independently, in any order, and in parallel. You can use a multipart upload for objects from 5 MB to 5 TB in size.
The article then starts you off with the requirements you'll need to follow along and some sample code to get the client up and running. The code then shows how to perform the file upload, making use of a file stream (via fopen) to pull in the contents of the file. The tutorial finishes by describing some of the other fields that can be used with the client and links to other SDKs that support this same functionality in other languages.