If you already have a SSL certificate installed properly on your website and still having problems communicating with CommBank Simplify's servers through PHP SDK or any other plugins and are seeing the following error, chances are you don't have an updated list of CA authorities in your execution environment.
— SSL certificate problem: self-signed certificate in certificate chain
It's a PHP/CURL level settings issue, and here is a support page for possible solutions.
1800 571 971
+61 7 3224 9711
7 a.m. - 7 p.m. AEST