php - interface covariance. How to Extend a Type?

I want to make a common interface namedValueObjectable with one method:of(), and implement the method with different argument types:int,string and so on.

How should I implement the interface ?

Example of the code:

    interface ValueObjectable
    {
        /**
         * @param $value
         * @return self
         */
        public static function of(mixed $value): self;
    }
    
    final class Sku implements ValueObjectable
    {
        public static function of(int $value = null): self
        {
            return new self($value);
        }
    }

    final class Ref implements ValueObjectable
    {
    
        public static function of(string $value): self
        {
            return new self($value);
        }
    }
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