PHP message: Notice: Undefined variable: DATABASE_USER_HOST

I’m getting this error even though I haven’t actually declared DATABASE_USER_HOST:

Notice: Undefined variable: DATABASE_USER_HOST in /home/jm4bwakj91is/public_html/student_dashboard.php on line 16

I double-checked the whole code, and I didn’t use DATABASE_USER_HOST. I also tried using error_reporting(E_ALL & ~E_NOTICE);.