Refactored example of Proxy pattern

Structural/Proxy/HeavyBankAccount.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace DesignPatterns\Structural\Proxy;
+
+class HeavyBankAccount implements BankAccount
+{
+    /**
+     * @var int[]
+     */
+    private $transactions = [];
+
+    public function deposit(int $amount)
+    {
+        $this->transactions[] = $amount;
+    }
+
+    public function getBalance(): int
+    {
+        // this is the heavy part, imagine all the transactions even from
+        // years and decades ago must be fetched from a database or web service
+        // and the balance must be calculated from it
+
+        return array_sum($this->transactions);
+    }
+}