Thank you for using Bitnami. I'm sorry but we do not support multiple PHP versions in our installers. The NGINX stack currently includes PHP 7.1. In order to understand your requirements better, could you please let us know why you need multiple PHP versions in your installation?
In case you are using your own PHP code, I suggest you post your question in a more specialized forum of PHP. This Community forum is intended to solve problems related to the Bitnami Stacks, Virtual Machine or Cloud Images.