php如何访问oracle存储过程？

create or replace package PKG_TRANS_REL is

 -- Author : test
 -- Created : 
 -- Purpose : test

 -- Public type declarations
 PKG_NAME varchar2(20) := 'PKG_TRANS_REL';
 --存储过程，测试用
 procedure pro_GC_withdraw(in_merch_no  in varchar2,
       in_withdraw_amt in number,
       out_result  out number,
       out_errmsg  out varchar2);
end PKG_TRANS_REL;

 $this->_pdo = new PDO(PDO_DB_DNS, PDO_DB_USER, PDO_DB_PASSWORD);
 $call = "CALL PKG_TRANS_REL.pro_GC_withdraw(?,?,?,?)";

 try{
   $stmt = $this->_pdo->prepare($call);

   $stmt->bindParam(1, $merch_no);
   $stmt->bindParam(2, $amount, PDO::PARAM_INT);

   $stmt->bindParam(3, $result, PDO::PARAM_INT, 4);
   $stmt->bindParam(4, $error_msg, PDO::PARAM_STR, 64);

   $stmt->execute();

  }catch (PDOException $e)
  {
   $msg = 'SQL:'.$e->getMessage();
   $msg = iconv('GBK','UTF-8',$msg);
   user_dump('SQL:'.$msg);
   return false;
  }

  ...