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It is possible to define constant values on a per-class basis remaining the same and unchangeable. Constants differ from normal variables in that you don't use the $ symbol to declare or use them.
The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call. It's also possible for interfaces to have constants. Look at the interface documentation for examples. As of PHP 5.3.0, it's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static).