异步请求PHP服务器，不返回数据？（附解决办法）

<form>
<input type="submit" value="提交" />
</form>

<script src="//cdn.bootcss.com/jquery/3.1.1/jquery.min.js"></script>

<script type="text/javascript">
    $(&#39;form&#39;).on(&#39;submit&#39;, function(e) {
        e.preventDefault();

        // 每隔一秒请求一次服务器
        var id = setInterval(function() {
            $.get(
                &#39;save.php?action=get&#39;,
                {},
                function(data) {
                    console.log(data);
                },
                &#39;json&#39;
            );
        }, 1000);

        $.post(
            &#39;save.php?action=post&#39;,
            {},
            function(data) {
                console.log(data);
                // 停止定时循环
                clearInterval(id);
            },
            &#39;json&#39;
        );
    });
</script>

<?php
session_start();$action = $_GET[&#39;action&#39;];
if ($action == &#39;post&#39;) {

   $_SESSION[&#39;time&#39;] = 0;
   session_write_close();

    while ($_SESSION[&#39;time&#39;] < 5) {
      session_start();
      $_SESSION[&#39;time&#39;] = $_SESSION[&#39;time&#39;] + 1;

      // 将SESSION数据写入文件中，并关闭写锁
      session_write_close();

      // sleep()模拟花费时间较长的程序，这样在关闭写锁之后，
      // 服务器就能够相应别的请求，如下的$action=get，
      sleep(1);
    }

    echo json_encode([session_id() => $_SESSION[&#39;time&#39;]]);
    exit();}if ($action == &#39;get&#39;) {
    echo json_encode([session_id() => $_SESSION[&#39;time&#39;]]);
    exit();}