Improve the backend and frontend of my website

Hello,

I have a website that needs improvement on the backend and frontend (more on the backend). We have a few bugs, and we would also like to have some functions added.
The website is built using mostly PHP.
The whole website is basically a ticket system, there are 2 types of users:
Users:
- Can open and reply to a ticket if it is opened
Admins:
- Can change status of a ticket into closed, open or delete the ticket.

Known bugs that needs fixing:
- Even tho credentials are correct, when trying to login it says invalid login and you have to try again.
- When an admin replies to a user's ticket it shows the user's name instead of the admin's name.

Functions that need to be added:
- Admins shouldn't be able to open a ticket.
- Instead of using an email address for logging in, you should be able to use an username to login and only use the email address for getting reset password emails.
- For now the users are only able to upload files to the server as attachments only when you start a ticket, but they should always be able to upload files as replies to a ticket
- Improve responsiveness of the website by adding small animations

Thank you.