PHP实例：PHP中ajax无刷新上传图片与图片下载功能

<meta charset="utf-8" > 
    <form id= "uploadForm">  
       <p >指定文件名： <input type="text" name="filename" value= ""/></p >  
        <p> 
         上传文件：  
         <input type="file" name="photo" onchange="showPreview(this)" class="file" /> 
         <img id="portrait" src="" width="70" height="75"> 
       </p>  
       <input type="button" value="上传" onclick="doUpload()" />  
    </form>  
    <script src="http://www.haoyunyun.cn/jquery.js"></script> 
    <script> 
    function doUpload() {  
       var formData = new FormData($( "#uploadForm" )[0]);  
       $.ajax({  
         url: 'submit.php' ,  
         type: 'POST',  
         data: formData,  
         async: false,  
         cache: false,  
         contentType: false,  
         processData: false,  
         success: function (returndata) {  
           alert(returndata);  
         },  
         error: function (returndata) {  
           alert(returndata);  
         }  
       });  
    }  
    </script> 
    <script type="text/javascript"> 
    function showPreview(source) { 
      var file = source.files[0]; 
      if (window.FileReader) { 
        var fr = new FileReader(); 
        fr.onloadend = function(e) { 
          document.getElementById("portrait").src = e.target.result; 
        }; 
        fr.readAsDataURL(file); 
      } 
    } 
   </script> 

<?php 
  if($_FILES['photo']['error']>0){ 
    echo "上传文件失败"; 
    die; 
  } 
  $dir='./photo/'; 
  $type=substr($_FILES['photo']['name'],strrpos($_FILES['photo']['name'],'.')); 
  $filename=time().rand(1000,9999).$type; 
  if(is_uploaded_file($_FILES['photo']['tmp_name'])){ 
    move_uploaded_file($_FILES['photo']['tmp_name'],$dir.$filename); 
    echo "上传成功"; 
  }else{ 
    echo "上传文件失败"; 
  }