为什么IE无法获取jQuery POST（）结果？

Question

I have a combination of jQuery POST() function with PHP.
jQuery:

token = $('input[name=signin-token]').val();
$.post('controller/create-captcha.php',{
     signin_token: token
}, function(data){
     $('#captcha').attr('placeholder',data);
});

PHP:

<?php
session_start();

if (isset($_POST['signin_token']) && isset($_SESSION['signin-token']) 
    && $_POST['signin_token'] == $_SESSION['signin-token']) {
    $rand1 = rand(1,10);
    $rand2 = rand(1,10);

    $_SESSION['captcha'] = $rand1+$rand2;
    echo $rand1.' + '.$rand2. ' =';
}
?>

This code is working well in both Chrome and Firefox. But IE can not fetch the result at all. What is the problem and how can I solve it?