public function getapiSignature()
 {
  $access_token=S('access_token');
  //先查询缓存中是否存在
  if($access_token)
  {
   $accesstoken=$access_token;
  }
  else//不存在则调用函数生成，并建立新的缓存
  {
   $accesstoken=getAccessToken($this->appid,$this->appsecret);
   S('access_token',$accesstoken,7200);
  }
    
     $apiticket=S('ticket');
     if($apiticket)
     {
      $ticket=$apiticket;
     }else
     {
      $ticket=getTicket($accesstoken);
      S('ticket',$ticket,7200);
     }
   
     $cur_url="http://www.dqiu.net/rentout";
    
     $apisignature=S('signature');
     if($apisignature)
     {
      $signature=$apisignature;
     }else
     {
      $signature=getSignature($ticket,$this->nonce,$this->timestamp,$cur_url);
            S('signature',$signature,7200);
     }
     return $signature;
 }