PHP user-defined constant can be like this:
define('APP',
[
'name'=>'Sadish',
'hobby'=>'Coding',
'occupation'=>'student'
]);
Here, APP is the user-defined constant which is defined using define() function available in PHP.
Also, Simply anything, like this :
define('APP','Demo PHP');
is a user-defined constant, which basically replaces any code available in PHP named as APP with the value 'Demo PHP'.