将iframe值检索到客户端页面

How can I retrieve values from the iframe form id, and call it to my client side html.

(client side html) sample.html

<head>
    <script>
        function alertThis(){
            alert(parent.myframe.formsubmit.a_token.value);
        }
    </script>
</head>

<body>

    <iframe name = "myframe" src="http://domain.com/sample/index.php">
    <button onClick = "alertThis();">Click Here</button>

</body>

(server side php) index.php

<form name = "formsubmit">
      <input id = "a_token"  value="<?php echo $_SESSION['user'] ?>"/>
</form>

but it does not return any.

var doc = window.frames['myframe'].document.getElementById('a_token').value; alert(doc);

try this.

Try this codepeace

<iframe name="myframe" id="myframe" src="http://domain.com/sample/index.php">

function alertThis(){
   var content = window.frames['myframe'].document.forms['formsubmit'].elements['a_token'].value;
   alert(content);
}

EDIT

Same issue has sorted using jQuery

Get radio value inside iframe

$('#myframe').contents().find('a_token').val()