test.php

/**
 * Confirmed Fatal errors by hand.
 *
 * Properties have to have typehint. Following is invalid.
 * public $array;
 *
 * Properties cannot have default. Following is invalid.
 * public $array = [];
 *
 * Child classes must also be readonly. Following is invalid.
 * $a = new class() extends A {};
 *
 */
readonly class A
{
    public array $array;
    public function __construct(array $array = [])
    {
        $this->array = $array;
    }

    public function get()
    {
        return $this->array;
    }
}

readonly class A1 extends A
{
    public \Closure $closure;
    public function __construct($array, \Closure $closure)
    {
        $this->closure = $closure;
        parent::__construct($array);
    }

    public function getProphecy() {
        return \call_user_func($this->closure);
    }

    public function get()
    {
        return $this->getProphecy()->makeProphecyMethodCall(__FUNCTION__, func_get_args());
    }
}