Hello!
First of all, welcome to the forums! I'm sure you'll like it here. I know I do.
Anyway, to your problem (although you should really post this in PHP Help, but I'll still try to help you out), you could store the username in a session variable, and make queries to the database to pull information through mysqli_fetch_array() using the username you stored. For example, if the username is "admin", store it in $_SESSION['user'] = "admin"; (given that you already started a session), then use that to fetch data from the database, in which that data would only be visible to the user "admin".
Sure, it isn't the safest way to do it, but that would be how the concept would go.