<?php

// i made some pretend subscriptions, but used the same variable name as you.
$allMfSubs = array(
	1 => array("id" => 1, "name" => "foo", "slug" => "1-foo", "for_sale" => true, "price" => 100),
	2 => array("id" => 2, "name" => "bar", "slug" => "2-bar", "for_sale" => true, "price" => 200),
	3 => array("id" => 3, "name" => "baz", "slug" => "3-baz", "for_sale" => true, "price" => 300),
	4 => array("id" => 4, "name" => "cux", "slug" => "4-cux", "for_sale" => true, "price" => 400),
	5 => array("id" => 5, "name" => "qux", "slug" => "5-qux", "for_sale" => true, "price" => 500),
);

// i used the same userIdArray name you picked, but i think subscriptionIds
// might be a more accurate name.
//
// it contains subscription id 3 and 5. you can change this to any number
// combination
$userIdArray = array(3, 5);

// i pass in all the subscriptions and the ids that i want
function user_active_subscriptions($allMfSubs, $userIdArray) {
	// i create an empty array. if the user has no active subscriptions, the
	// function will return an empty array/no subscriptions
	$activeSubscriptions = array();

	// i loop through the subscription ids
	foreach($userIdArray as $subscriptionId) {
		// if the subscriptionId isn't found in $allMfSubs, the
		// $subscription variable will have the value of null
		$subscription = $allMfSubs[$subscriptionId];

		// an if statement will treat null and false the same way.
		// if the $subscription is true/not null, add it to the array of
		// active subscriptions
		if($subscription) {
			$activeSubscriptions[] = $subscription;
		}
	}

	// return whatever you've found
	return $activeSubscriptions;
}

// because the $allMfSubs indexes match to the nested array's id, I only care if
// the $allMfSubs index exists. i dont have to care about any of the values in
// the nested arrays

$results = user_active_subscriptions($allMfSubs, $userIdArray);
print_r($results);