Refactored example of Proxy pattern

Structural/Proxy/BankAccountProxy.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace DesignPatterns\Structural\Proxy;
+
+class BankAccountProxy extends HeavyBankAccount implements BankAccount
+{
+    /**
+     * @var int
+     */
+    private $balance;
+
+    public function getBalance(): int
+    {
+        // because calculating balance is so expensive,
+        // the usage of BankAccount::getBalance() is delayed until it really is needed
+        // and will not be calculated again for this instance
+
+        if ($this->balance === null) {
+            $this->balance = parent::getBalance();
+        }
+
+        return $this->balance;
+    }
+}