I am trying to figure out how to properly document some code, mostly so that tools like PHPStan and Psalm don’t complain.

Simplified, I have an abstract class that defines methods for setting and getting the state of child classes. The type of that state might be different child class to child class, e.g. a string in one case, an array in another, etc..

In reading up on this, I figured the way to accomplish it would be to use @template on the base class, and more specific param/return types on the children. i.e.:

/**
 * The base class, where I use a template and "mixed" param/return types.
 *
 * @template StateType
 */
abstract class hasState {
    
    /**
     * @param StateType $state
     */
    public function setState(mixed $value): void;

    /**
     * @return StateType
     */
    public function getState(): mixed;

}

/**
 * A child class, where the state is a string.
 */
class stringState implements hasState {

    protected string $state = '';

    /**
     * @param string $value
     */
    public function setState(string $value): void
    {
        $this->state = $value;
    }

    /**
     * @return string
     */
    public function getState(): string
    {
        return $this->state;
    }
}

/**
 * Another child class, where the state is an array.
 */
class arrayState implements hasState {

    protected array $state = [];

    /**
     * @param array $value
     */
    public function setState(array $value): void
    {
        $this->state = $value;
    }

    /**
     * @return array
     */
    public function getState(): array
    {
        return $this->state;
    }
}

In short, this doesn’t work. I get errors like this in Psalm:

Declaration must be compatible with hasState->setState(state: mixed)

psalm: MethodSignatureMismatch: Argument 1 of stringState::setState has wrong type ‘string’, expecting ‘mixed’ as defined by hasState::setState

psalm: MoreSpecificImplementedParamType: Argument 1 of stringStage::setState has the more specific type ‘string’, expecting ‘mixed’ as defined by hasState::setState

Is there no way to accomplish what I’m trying to do: i.e. document and type the params/return values in child classes to use more specific scalar types than the parent?