The bid is just a requirement to contact you at this stage. As you know (therefore your instruction about not bidding yet) the information posted is too general to provide a bid and a timeframe.
I have a lot of experience developing back ends and have my own code libraries but I can use the PHP framework of your choice if the use of a framework is a requirement. User authentication and authorization, role based security (admin, user, visitor, etc.) social logins, APIs to provide designers with the data they need (usually in json format) via ajax calls and payment platforms integration are the standard these days. I can do that and more delivering clean, object oriented, well commented PHP code.
I have been developing web applications for more than 20 years. You can see two recent projects on my profile page with a screen captures and brief descriptions. Two days form now (on monday) I will finish my most recent job: a web application to monitor tank sensors and administer clients, locations, sensors, alarms, etc. I will update my portfolio here at freelancer next week (I always wait for the project to be delivered and request permission from the client).
Please provide detailed information about the project including your deadline, I will be happy to place a real bid.