About This will be a basic chat application backend using Slim Framework 4 and Sqlite3.

Initial Plan for models and actions

MODELS

AuthToken:
    - id: primary_key
    - user: foreign key
    - token
    - expires_at

User:
    - id: primary_key
    - username: unique
    - email
    - created_at
    - password_hash

Chat:
    - id: primary_key
    - user1
    - user2

    - unique together: user1, user2

Message:
    - id: primary_key
    - chat: Chat foreign key
    - sender: userforeign key
    - content
    - created_at
    - is_seen
    - reply_to?: Message foreign key

ENDPOINTS

register - POST - register/
    - username
    - password

    - returns ok

login - POST - login/
    - username
    - password

    - returns auth_token

listChats - GET - chat/
    - Returns paginated list of chats

getChat - GET - chat/{username}
    - Returns paginated list of messages

sendMessage - POST chat/{username}/message
    - message_content
    - reply_to?: message_id

    - Returns generated message_id

markMessageAsSeen - POST - chat/{username}/mark-as-seen
    - Returns ok

deleteMessage - DELETE - chat/{username}/message/{message_id}
    - Returns ok