The AWS SDK for PHP provides support for the multipart upload API (see Uploading Objects Using Multipart Upload API).
The AWS SDK for PHP high-level abstractions simplify the multipart upload flow. In just a few lines of code you can upload files to Amazon S3. This is recommended for simple file uploads.
The AWS SDK for PHP low-level APIs correspond to the multipart upload REST operations (see Using the REST API for Multipart Upload). Use the low-level API when you need to pause and resume multipart uploads, vary part sizes during the upload, or do not know the size of the data in advance. If you do not have these requirements, use the high-level API.