is_numeric_array_key example

    public function register($hook, $callback, $priority = 0) {
        if (is_string($hook) === false) {
            throw InvalidArgument::create(1, '$hook', 'string', gettype($hook));
        }

        if (is_callable($callback) === false) {
            throw InvalidArgument::create(2, '$callback', 'callable', gettype($callback));
        }

        if (InputValidator::is_numeric_array_key($priority) === false) {
            throw InvalidArgument::create(3, '$priority', 'integer', gettype($priority));
        }

        if (!isset($this->hooks[$hook])) {
            $this->hooks[$hook] = [
                $priority => [],
            ];
        } elseif (!isset($this->hooks[$hook][$priority])) {
            $this->hooks[$hook][$priority] = [];
        }