使用ajax和php上传，无需iFrame或flash引擎

我正在尝试制作一个带有进度条的ajax上传功能，我试图简化我在互联网上找到的一个例子，我得到了这个功能：

<form>
        <input type="file" id="file" /><label for="file"></label><label id="aaa"></label>
        <button id="sub">abc</button>
    </form>
    <script src="js/jquery-1.7.1.min.js"></script>
    <script>
    function uploadFile(files) {
        var xmlhttp;
        if(window.XMLHttpRequest)
            xmlhttp = new XMLHttpRequest();
        else
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        xmlhttp.upload.onprogress = function(e) {
            $("#aaa").empty().append(e.loaded + " - " + e.total);
        }
        xmlhttp.upload.onload = function(e) {
            $("#aaa").empty().append("finish");
        }
        xmlhttp.onreadystatechange = function() {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                alert(xmlhttp.responseText);
            }
        }
        xmlhttp.open("post", "post.php", true);
        xmlhttp.setRequestHeader("If-Modified-Since", "Mon, 26 Jul 1997 05:00:00 GMT");
        xmlhttp.setRequestHeader("Cache-Control", "no-cache");
        xmlhttp.setRequestHeader("X-Requested-With", "XMLHttpRequest");
        xmlhttp.setRequestHeader("X-File-Name", files[0].fileName);
        xmlhttp.setRequestHeader("X-File-Size", files[0].fileSize);
        xmlhttp.setRequestHeader("Content-Type", "multipart/form-data");
        xmlhttp.send(files[0]);
    }
    $(document).ready(function() {
        $("#file").change(function() {
            uploadFile(this.files);
        });
    });
    </script>

当我上传接近或低于800kb-1MB的文件时，它工作得很好，速度很快，但当我试图上传更大的文件时它会出错。进度条显示它正在上传，但onreadystatechange不会触发，文件也不会出现在服务器上。

回复这个ajax的php是这样的：

<?php
// e.g. url:"page.php?upload=true" as handler property
//print_r($_SERVER);
if(
    // basic checks
    isset(
        $_SERVER['CONTENT_TYPE'],
        $_SERVER['CONTENT_LENGTH'],
        $_SERVER['HTTP_X_FILE_NAME']
    ) &&
    $_SERVER['CONTENT_TYPE'] == 'multipart/form-data'
){
    // create the object and assign property
    /*$file = new stdClass;
    $file->name = basename($_SERVER['HTTP_X_FILE_NAME']);
    $file->size = $_SERVER['HTTP_X_FILE_SIZE'];
    $file->content = file_get_contents("php://input");
    // if everything is ok, save the file somewhere
    if(file_put_contents('files/'.$file->name, $file->content))
        echo "OK";*/
        $file->name = basename($_SERVER['HTTP_X_FILE_NAME']);
        $input = fopen('php://input', 'rb');
$file = fopen('files/'.$file->name, 'wb');
stream_copy_to_stream($input, $file);
fclose($input);
fclose($file);
} else {
    // if there is an error this will be the output instead of "OK"
    echo "Error";
}
?>

提前谢谢你，丹尼尔！