如何将配置文件中定义的$ admin转换为mysql变量

Currently, on a forum I am making, I have just defined

$admin='adminaccount';

However, I want to make the admin perk a variable on the database.

I currently use:

if(isset($_SESSION['username']) and $_SESSION['username']==$admin)

but, I have no idea how to turn this into a varible on the mysql.