Add PHPUnit example

php/phpunit/tests/CalculatorTest.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests;
+
+use App\Calculator;
+use PHPUnit\Framework\TestCase;
+
+class CalculatorTest extends TestCase
+{
+    public function test_add(): void
+    {
+        $calculator = new Calculator();
+
+        $this->assertSame(3, $calculator->add(1, 2));
+    }
+}