Why can only use constants for attribute fields in the interface?-php Tutorial

Why can I only use constants for attribute fields in interfaces? why can't I use $? how can I use variables?

Reply to discussion (solution)

When defining a class, the attribute must be a known amount, and the interface is no exception

Interfaces are "protocols" or "rules" in reality ". You can see who can change the cables in the USB interface of the mobile phone.

Let alone Apple ........

When defining a class, the attribute must be a known amount, and the interface is no exception

You can also declare a variable, for example, $ abc in the class.

You should first clarify the attributes and variables!
The same statement has different meanings in different places.

Post your code, which is the basis for discussion