PHP Adding Two arrays

I'm currently attempting to produce a progress bar which shows how much a user has spent in comparison to their maximum allowance. I have calculated the sum of the user's costs, as given by $sumOfCosts, however, I'm attempting to subtract the $sum of costs from the user's maximum allowance however, the code below doesn't work. Does anyone have any advice?

appQueries.php

<?php

class appQueries {
    protected $db = null;
    public function __construct($db){
          $this->db = $db;
    }

    public function costsSum($user_id){
        $query = "SELECT SUM(value) AS costSum FROM costs";
        $pdo = $this->db->prepare($query);
        $pdo->bindParam(':user_id', $user_id);
        $pdo->execute();
        return $pdo->fetch(PDO::FETCH_ASSOC);
    }

    public function getMaxAmount($userid){
        $query = "SELECT maxAmount FROM users WHERE user_id = :userid";
        $pdo = $this->db->prepare($query);
        $pdo->bindParam(':userid', $userid);
        $pdo->execute();
        return $pdo->fetch(PDO::FETCH_ASSOC);
    }
}

homepage.php

$appQueriesObject = new appQueries($DBH); 
$maximumCost = $appQueriesObject->getMaxAmount($_SESSION['userData']['user_id']);
$sumOfCosts = $appQueriesObject->costsSum($_SESSION['userData']['user_id']);
$amountRemaining = array($maximumCost + $sumOfCosts);

echo $maximumCost['user_max_amount'];
echo array_sum($sumOfCosts);
echo $amountRemaining;