Can't use constants with empty()?

Question

parent::_initialize();//The initialization operation in the inherited parent class is different from what the teacher said

In addition, if(empty(USER_ID)) reports an error, if(USER_ID!=null )Can.

Answer 1

bool empty ( mixed $var )

Determines whether a variable is considered empty. When a variable does not exist, or its value is equal to FALSE, then it is considered not to exist. empty() does not generate a warning if the variable does not exist.

It means that empty is used to detect variables; Attached link

Answer 2

empty() detects variables, right