Add k3s cluster expansion documentation and scripts

- Complete expansion guide for 2/4/6 node scenarios
- Quick join scripts for worker and master nodes
- Health check and diagnostic scripts
- Quick reference card for common operations

php-test/Database.php

@@ -0,0 +1,22 @@
+<?php
+require_once 'config.php';
+
+class Database {
+    private static ?PDO $instance = null;
+
+    public static function getConnection(): PDO {
+        if (self::$instance === null) {
+            try {
+                $dsn = "mysql:host=" . DB_HOST . ";dbname=" . DB_NAME . ";charset=" . DB_CHARSET;
+                self::$instance = new PDO($dsn, DB_USER, DB_PASS, [
+                    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
+                    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
+                    PDO::ATTR_EMULATE_PREPARES => false,
+                ]);
+            } catch (PDOException $e) {
+                die("数据库连接失败: " . $e->getMessage());
+            }
+        }
+        return self::$instance;
+    }
+}