我先帖代码了，其他没啥意思。

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Wechat extends MY_Controller 
{
	public function __construct()
	{
		parent::__construct();
	}

	public function valid()
    {
        $echoStr = $this->input->get('echostr');
        //valid signature , option
        if($this->checkSignature()){
        	echo $echoStr;
        	exit;
        }
    }

    public function responseMsg()
    {
		//get post data, May be due to the different environments
		$postStr = $GLOBALS["HTTP_RAW_POST_DATA"];

      	//extract post data
		if (!empty($postStr)){
                
              	$postObj = simplexml_load_string($postStr, 'SimpleXMLElement', LIBXML_NOCDATA);
                $fromUsername = $postObj->FromUserName;
                $toUsername = $postObj->ToUserName;
                $keyword = trim($postObj->Content);
                $time = time();
                $textTpl = "<xml>
							<ToUserName><![CDATA[%s]]></ToUserName>
							<FromUserName><![CDATA[%s]]></FromUserName>
							<CreateTime>%s</CreateTime>
							<MsgType><![CDATA[%s]]></MsgType>
							<Content><![CDATA[%s]]></Content>
							<FuncFlag>0</FuncFlag>
							</xml>";             
				if(!empty( $keyword ))
                {
              		$msgType = "text";
                	$contentStr = "Welcome to wechat world!";
                	$resultStr = sprintf($textTpl, $fromUsername, $toUsername, $time, $msgType, $contentStr);
                	echo $resultStr;
                }else{
                	echo "Input something...";
                }

        }else {
        	echo "";
        	exit;
        }
    }
		
	private function checkSignature()
	{
        $signature 	= $this->input->get('signature');
        $timestamp 	= $this->input->get('timestamp');
        $nonce 		= $this->input->get('nonce');
        		
		$token = TOKEN;
		$tmpArr = array($token, $timestamp, $nonce);
		sort($tmpArr);
		$tmpStr = implode( $tmpArr );
		$tmpStr = sha1( $tmpStr );
		
		if( $tmpStr == $signature ){
			return true;
		}else{
			return false;
		}
	}

}

这个和官方一样，没啥改动的。放到你的回调地址就好。

还有一个地方，要注意，就是token这个放好。

defined('TOKEN')      		   OR define('TOKEN', 'caisangzi');

把TOKEN放到constants.php里面。