Caput Variable in Array

I have this Array, $rule

WC_Memberships_Membership_Plan_Rule Object
(
    [data:WC_Memberships_Membership_Plan_Rule:private] => Array
        (
            [membership_plan_id] => 513
            [id] => rule_5ac4cbbe44f46
            [object_ids] => Array
                (
                    [0] => 385
                )

            [discount_type] => amount
            [discount_amount] => 1.78
            [active] => yes
            [rule_type] => purchasing_discount
            [content_type] => post_type
            [content_type_name] => product
            [rule_key] => 1
        )

)

I need to capture the value of [membership_plan_id] but I tried everything already. Another option would be to use array_key_exists('membership_plan_id',$rule) but it also did not work.