PHPUnit如何mock函数内部实例化的对象

class cash{    public function __construct(){}    public function isCashed(){       //do something        return 1;    }}class sendMsg{    public function send(){          $cash = new cash();          $isCashed = $cash->isCashed();          return $isCashed;         //do something    }}

class sendMsgTest extension PHPUnit_Framework_TestCase{             public function testSend(){             $sendMsg = new sendMsg();                          //如何mock一个cash      }}