处理PHP上传文件错误代码从和AJAX请求/响应没有JQuery

我想在不选择文件的情况下提交表单，通常我得到php错误码4 =>没有从$_FILES['file']['error']上传的文件一切都好，如果我只使用HTML和php工作，但不是当我添加AJAX !

My problems:

• 我不知道如何发布/发送空文件数组才能处理
• 我不知道如何写一个响应回AJAX脚本，它应该是在进行处理程序，或错误处理程序或我不知道…!
• 我不想用JQuery工作，我复制/粘贴我所有的代码来构建任何东西，因为我在编码0级，所以我需要理解每一行和JQuery对我来说就像星际空间!

格式:

<form id="upload_form" enctype="multipart/form-data" method="post" >
    <input type="file" name="file" id="file_id" /><br />
    <input type="button" value="Upload File" onclick="uploadFile()" /><br />
</form> 

<script>
function uploadFile(){
    // target the file that is to be uploaded
    var file = document.getElementById("file_id").files[0];
    // var file = document.getElementById("file_id"); is not working,
    // what is files ? why do i need .files[0]?
    // Create a new formdata object instance
    var formdata = new FormData();
    // append the file to the formdata
    formdata.append("file", file); 
    // on submit, if no file selected, the error index is 4 in php
    // do i have to append the file error here ?
    // Build the AJAX request
    var xhr = new XMLHttpRequest(); 
    // add event listeners
    // progress handler 
    xhr.upload.addEventListener("progress", progress_handler, false);
    // complete handler        
    xhr.addEventListener("load", complete_handler, false);
    // error handler
    xhr.addEventListener("error", error_handler, false);
    // abort handler
    xhr.addEventListener("abort", abort_handler, false);

    // open a php script
    xhr.open("POST", "file_upload.php");
    // send the AJAX request
    xhr.send(formdata);
}
function progress_handler(event){  
    // create a progress bar, or a % of uploading...
    // using event.loaded and event.total (returns size in bytes)
}
// when the operation is finished
function complete_handler(event){
    // the message php is echoing will be put inside that div
    // using responseText !
    document.getElementById("some_div").innerHTML = event.target.responseText;
    // can we echo back other things beside strings, like an array, int... ?
    //set the progress bar to full
}
function error_handler(event){
    document.getElementById("status").innerHTML = "Upload failed !";
    // do i need to append the error here instead ? how ?
    // nothing that i tried around $_FILES[file]['error'] in php worked back here !
}
function abort_handler(event){
    //cancel listener if the file upload is aborted
    document.getElementById("status").innerHTML = "Upload aborted !";
}
</script>

PHP文件:file_upload.php

<?php
echo '<pre>';  print_r($_FILES);  echo '</pre>';
// this is working when file is selected and echo back all array elements
// but why there NOTHING returned when no file selected, not even en empty array !!!
// -----------------------------------------------------
if(isset($_FILES['file']) AND $_FILES['file']['error'] == 0) 
{
    // move the file from the tmp folder to upload folder in website
}
else
{
    echo "Error : Please select a file before clicking the Upload button !";
    // i don't want this which is working!
    echo $_FILES["file"]["error"]; 
    // i want this which is working only without AJAX !!!
    // and would be : 
    // 1 > The uploaded file exceeds the upload_max_filesize directive in php.ini
    // ...
    // 4 > No file was uploaded at all !
    // until 
    // 8 > a php extension stopped the upload...
}
?>

非常感谢。