PHP AJAX 文件上传

i've looked all over stackoverflow and the general internet and i can't find a simple AJAX file uploader (Just form submission). The ones i have found aren't flexible enough and don't fit my purpose. If i could ajust this script to file uploads, that would be great:

<script type="text/javascript">
 function upload(str)
 {
 if (str.length==0)
 { 
document.getElementById("txtHint").innerHTML="";
 return;
  }
 if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
   xmlhttp=new XMLHttpRequest();
   }
 else
   {// code for IE6, IE5
   xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
   }
 xmlhttp.onreadystatechange=function()
   {
   if (xmlhttp.readyState==4 && xmlhttp.status==200)
     {
     document.getElementById("hint").innerHTML=xmlhttp.responseText;
     }
   }
 xmlhttp.open("GET","ajax/link.php?q="+str,true);
 xmlhttp.send();
 }

在 HTML 中我会有

<input type='file' onblur='upload(this.value)'>