Update tests to work with PHPUnit 10

tests/User.php

@@ -0,0 +1,28 @@
+<?php declare(strict_types=1);
+
+namespace Helper;
+
+// Check interface exists to fix "Fatal error: Interface 'JsonSerializable' not found in ../tests/PHPCurlClass/User.php
+// on line X".
+if (interface_exists('JsonSerializable')) {
+    class User implements \JsonSerializable
+    {
+        private $name;
+        private $email;
+
+        public function __construct($name = null, $email = null)
+        {
+            $this->name = $name;
+            $this->email = $email;
+        }
+
+        #[\ReturnTypeWillChange]
+        public function jsonSerialize()
+        {
+            return [
+                'name' => $this->name,
+                'email' => $this->email,
+            ];
+        }
+    }
+}