How to declare a abstract const in php?

@darion 

In PHP, it is not possible to declare an abstract constant directly. However, you can achieve something similar by using a combination of an abstract constant method and a private constant value. Here's an example:

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abstract class MyClass {
    private const MY_CONSTANT = 'abc';

    abstract protected static function getConstantValue(): string;

    public static function useConstant(): string {
        return static::getConstantValue();
    }
}

class MyChildClass extends MyClass {
    protected static function getConstantValue(): string {
        return self::MY_CONSTANT;
    }
}

echo MyChildClass::useConstant(); // Output: abc

In this example, the MyClass abstract class defines a private constant MY_CONSTANT. It also declares an abstract method getConstantValue() which should return the value of the constant. The useConstant() method can be used to access the value of the constant through the getConstantValue() method.

The MyChildClass extends MyClass and implements the getConstantValue() method by returning the value of the private constant.

Note: Using this approach, the constant can still be accessed outside of the class hierarchy using the useConstant() method. But the actual constant value is encapsulated and cannot be directly accessed or modified outside the class hierarchy.