Added php project and its unit tests.

tests/SimpleTest.php

@@ -0,0 +1,24 @@
+<?php
+
+require_once 'src/Simple.php';
+
+use PHPUnit\Framework\TestCase;
+
+class SimpleTest extends TestCase
+{
+    public function testAdd()
+    {
+        $simple = new Simple();
+        $result = $simple->add(2, 3);
+        $this->assertEquals(5, $result);
+    }
+
+    public function testSubtract()
+    {
+        $simple = new Simple();
+        $result = $simple->subtract(3, 2);
+        $this->assertEquals(1, $result);
+    }
+}
+
+?>