add singleton pattern in Db service of getPdo() function

Westdc/Db/Dbh.php

@@ -11,7 +11,7 @@ class Dbh
 	function __construct($db = NULL)
 	{
         if($db == NULL)
-		    $this->db = new PDO();
+		    $this->db = PDO::getInstance();
         else
             $this->db = $db;
 	}

Westdc/Db/Pdo.php

@@ -3,26 +3,31 @@ namespace Westdc\Db;
 
 use Zend\Config\Config as Zend_Config;
 
-class Pdo extends \PDO
+class Pdo
 {
-	private $debug = 0;		//调试模式
+
+    private static $_instance = NULL;
 	
-	private $config_local_path = "config/autoload/local.php";
-	
-	function __construct($DSN = NULL)
+	private function __construct($DSN = NULL)
     {
-        if (!empty($DSN)) {
-            parent::__construct($DSN);
-        } else {
-            $config_local = new Zend_Config(include $this->config_local_path);
+
+    }
+
+    public static function getInstance()
+    {
+        if (self::$_instance === null) {
+            $config_local = new Zend_Config(include "config/autoload/local.php");
 
             $dsn = "pgsql:host={$config_local->db->hostname};"
                 . "port={$config_local->db->port};"
                 . "dbname={$config_local->db->database};"
                 . "user={$config_local->db->username};"
                 . "password={$config_local->db->password}";
-            parent::__construct($dsn);
+            self::$_instance = new \PDO($dsn);
         }
+
+        return self::$_instance;
     }
 
+
 }

Westdc/Member/Cookie.php

@@ -2,7 +2,7 @@
 namespace Westdc\Member;
 
 use Westdc\Helpers\Config;
-use Westdc\Db\PDO as Db;
+use Westdc\Db\Pdo as Db;
 
 class Cookie
 {
@@ -27,7 +27,7 @@ class Cookie
 
 	function __construct()
 	{
-		$this->db = new Db();
+		$this->db = Db::getInstance();
 		$this->config = Config::get();
 		
 		if(!empty($_COOKIE['scr']))

Westdc/Service/ServiceAgent/Db.php

@@ -21,7 +21,7 @@ class Db {
 
     public function getPdo()
     {
-        return new WestdcDb\Pdo;
+        return WestdcDb\Pdo::getInstance();
     }
 
     public function getDbh()