It is now June 2025 and this problem persists. The Polly section of the AWS PHP SDK appears to have been last updated in June 2016. The PHP SDK is broken for the DescribeVoices command (it does not include the SupportedEngines field in the result) and the SynthesizeSpeech command (it drops or ignores the Engine parameter).

I had to fall back to invoking the AWSCLI using Exec(), which does work as expected.

I am using PHP v8.2 and installed the AWS PHP SDK yesterday. (June 9 2025).