<form enctype="multipart/form-data" action="upload.php?action=img" method="post" name="imge">

     <input value="导入文件" type="file" name="img" id="file" />

     <input type="submit" id="imgbut" class="btn" value="上传图片" />

 </form>

 if ($_FILES["img"]["error"] > 0) {

         switch ($_FILES["img"]["error"]) {

             case 1 :

                 echo "<script type='text/javascript'>alert('上传的文件超过了 php.ini 中 upload_max_filesize 选项限制的值<br>');history.back();</script>";

                 break;

             case 2 :

                 echo "<script type='text/javascript'>alert('上传文件的大小超过了 HTML 表单中 MAX_FILE_SIZE 选项指定的值');history.back();</script>";

                 break;

             case 3 :

                 echo "<script type='text/javascript'>alert('文件只有部分被上传');history.back();</script>";

                 break;

             case 4 :

                 echo "<script type='text/javascript'>alert('没有文件被上传');history.back();</script>";

                 break;

             default :

                 echo "<script type='text/javascript'>alert('末知错误');history.back();</script>";

         }

         exit;

     }

     $maxsize = 1000000;

     //step 2 使用$_FILES["pic"]["size"] 限制大小 单位字节 2M=2000000

     if ($_FILES["img"]["size"] > $maxsize) {

         echo "<script type='text/javascript'>alert('上传的文件太大，不能超过{$maxsize}字节');history.back();</script>";

         exit;

     }

     //step 3 使用$_FILES["pic"]["type"]或是文件的扩展名 限制类型 MIME image/gif image/png  gif png jpg

     $allowtype = array("png", "gif", "jpg", "jpeg");

     $arr = explode(".", $_FILES["img"]["name"]);

     $hz = $arr[count($arr) - 1];

     if (!in_array($hz, $allowtype)) {

         echo "<script type='text/javascript'>alert('这是不允许的类型');history.back();</script>";

         exit;

 }

 $filepath = "uploads/";

     //为了符合UBB的路径

     $randname = date("Y") . date("m") . date("d") . date("H") . date("i") . date("s") . rand(100, 999) . "." . $hz;

     //将临时位置的文件移动到指定的目录上即可

     if (is_uploaded_file($_FILES["img"]["tmp_name"])) {

         if (move_uploaded_file($_FILES["img"]["tmp_name"], $filepath . $randname)) {

             echo "<script type='text/javascript'>alert('上传成功');history.back();</script>";

             session_start();

             $_SESSION['images'] = $filepath . $randname;

             //echo $_SESSION['images'];

         } else {

             echo "<script type='text/javascript'>alert('上传失败');history.back();</script>";

         }

     } else {

         echo "<script type='text/javascript'>alert('不是一个上传文件');history.back();</script>";

 }