会话上传进度，Ajax调用和空$ _SESSION

My goal is to, upon clicking the form submit button, to upload the attachments from the form to the server and show a progress bar of that happening and then submitting the form (ie. mailing the message).

upload_form.php:

<form action="email_message.php" method="post" enctype="multipart/form-data" name="fileform" id="fileform">
    <input type="hidden" name="MAX_FILE_SIZE" value="50000000"/>
    <input type="hidden" name="<?php echo ini_get("session.upload_progress.name"); ?>" value="uploads"/>
    <label for="userfile1">Upload a file:</label>
    <input type="file" name="userfile1" id="userfile1" class="userfile"/>   
    <input id="submit_btn" type="submit" value="Send Message"/>
</form>

In the same page, I run the following code to prevent the form from being executed and sending a request to upload all of the files from the form.

$(document).ready(function(){
    $("#fileform").submit(function(e){
        e.preventDefault();
        var self = this;

        var formData = new FormData(document.getElementById("fileform"));
        var upload_req = $.ajax({
            url: "./upload_multiple_files.php",
            type: "POST",
            data: formData,
            processData: false,
            contentType: false
        });
        upload_req.done(function(data){
            alert("Uploading complete!");
        });
    });
});

upload_multiple_files.php:

<?php
session_start();

foreach ($_FILES as $k => $v){
    // code to deal with file errors

    if (is_uploaded_file($v['tmp_name'])){
        // code to rename file

        echo "<p>The file was successfully uploaded.</p>";
    }else{
        echo "<p>The file was not uploaded.</p>";
    }
}
?>

All of this works: the files are all uploaded to the server.

The problem I have is integrating PHP Upload Session Progress (http://php.net/manual/en/session.upload-progress.php).

I know I need to use session.upload_progress.name and the $_POST array to get the file upload information but I'm not sure where to place it. I want to create an ajax call with an interval to periodically get the upload progress to be displayed on my form page. However, when I create a new page, the session information is empty. Here is an example of a page I tried:

get_progress.php:

<?php
session_start();

// $key is a combination of session.upload_progress.prefix and session.upload_progress.name

$results = array("content_length" => $_SESSION[$key]['content_length'],
    "bytes_processed" => $_SESSION[$key]['bytes_processed']
);

echo json_encode($results);
?>

I checked the session ids from upload_form.php and get_progress.php and they are the same.

Any reason why $_SESSION is empty in get_progress.php? I think I missed something easy but I can't figure it out.

@Perry Your answer is right, session.upload_progress.enabled wasn't enabled in php.ini. Thanks.