ajax - xmlhttprequest send json在php端接受数据的问题

<code class="lang-javascript">    var uinfo   = {};
    var uname   = document.getElementById('uname'); 
    var upwd    = document.getElementById('upwd');  
    uinfo['uname']  = uname.value;
    uinfo['upwd']   = upwd.value; 
    var usent = JSON.stringify(uinfo);
    var xhr = null;
    if(window.XMLHttpRequest){
        xhr = new XMLHttpRequest();
    }else{
        xhr = ActiveXObject('Microsoft.XMLHttp');
    }
    xhr.open("POST", "/controler/login.php",true);
    //etc
     xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xhr.send(usent);
</code>

<code class="lang-javascript">    var uinfo   = {};
    var uname   = document.getElementById('uname'); 
    var upwd    = document.getElementById('upwd');  
    uinfo['uname']  = uname.value;
    uinfo['upwd']   = upwd.value; 
    var usent = JSON.stringify(uinfo);
    var xhr = null;
    if(window.XMLHttpRequest){
        xhr = new XMLHttpRequest();
    }else{
        xhr = ActiveXObject('Microsoft.XMLHttp');
    }
    xhr.open("POST", "/controler/login.php",true);
    //etc
     xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xhr.send(usent);
</code>

<code>xhr.send('json='+usent);
</code>