为什么我的PHPworkman程序中医用json_decode 就发生错误

$ws_worker->onMessage = function($connection, $data)
{
    // 向客户端发送hello $data
    $m=Array("type"=>"login","ss"=>"sd");
    $m=json_encode($m);
    echo $data;
    echo $m;
    echo json_decode($m);

    broadcast($data);
};