Is it permissible to declare a method with a privacy modifier inside an interface?

In PHP, interfaces are intended for declaring public methods that must be implemented by classes. Declaring methods with a private modifier inside an interface is not allowed — all interface methods are public by default.

Attempting to declare a private method in an interface will result in an error.

An example of a correct interface:

interface Logger {
    public function log(string $message);
}

If private logic is needed, it is implemented inside the class, and the interface only describes the public contract.