Totally agree with @Andreas. When you build a modern application that supports on the different platforms such as web or mobile, you need to design a standard API and then next step is to build a web application/mobile app to consume this API. And because of all the web/devices are all consuming from the same API, they are interacted with the same database. Unless they are using different API.
Can refer to the image below for easy understanding.

So talk about how authentication and authorization can be done from the mobile app or from the web app. You should take a look at OAuth2. It is a protocol for securing API services from untrusted devices, and it provides a nice way to authenticate mobile users via what is called token authentication.
The workflow will look like below, on both web and mobile app.
A user opens up your mobile app and is prompted for their username or email and password.
You send a POST request from your mobile app to your API service with the user’s username or email and password data included (OVER SSL for sure. If you don't know about it, google it).
You validate the user credentials, and create an access token for the user that expires after a certain amount of time.
You store this access token on the mobile device, treating it like an API key which lets you access your API service.
Once the access token expires and no longer works, you re-prompt the user for their username or email and password.
Reference
REST API from PHP
The ultimate guide for Mobile Security