Build a risk tool

We have developed a tool using Google Sheets/MS Excel which can be used to assess an organisation's privacy risks. The tool works by asking users for information about their organisation and how they handle personal data, and by then using formulas it identifies pre-populated risks that might apply in the circumstances. The tool also makes use of formulas to hide/show questions depending on previous responses. We would like to turn this tool into a website. The website will require membership functionality which allows users to sign up and authenticate, and subscription functionality as this will be a paid service. We have an existing tool which offers membership functionality and is built using PHP - ideally, we would use this as a starting point for the new tool. The website will need to allow users to input information and, using branching logic, ask additional questions if required before displaying a list of generated risks. Ideally, it should be possible for an administrator to edit and create new questionnaires and risks, which would also require an interface for editing the branching logic.