图片上传PHP

这是我的代码;

<?php

 define ("MAX_SIZE","500"); 

 function getExtension($str) {
         $i = strrpos($str,".");
         if (!$i) { return ""; }
         $l = strlen($str) - $i;
         $ext = substr($str,$i+1,$l);
         return $ext;
 }

 $errors=0;
 if(isset($_POST['Submit'])) 
 {

    $image=$_FILES['image']['name'];
    if ($image) 
    {
        $filename = stripslashes($_FILES['image']['name']);

        $extension = getExtension($filename);
        $extension = strtolower($extension);

 if (($extension != "jpg") && ($extension != "jpeg") && ($extension != "png") && ($extension != "gif")) 
        {
        //print error message
            echo '<h1>Unknown extension!</h1>';
            $errors=1;
        }
        else
        {

 $size=filesize($_FILES['image']['tmp_name']);

if ($size > MAX_SIZE*1024)
{
    echo '<h1>You have exceeded the size limit!</h1>';
    $errors=1;
}

$image_name=time().'.'.$extension;
$newname="user_img/".$image_name;
$copied = copy($_FILES['image']['tmp_name'], $newname);
if (!$copied) 
{
    echo '<h1>Copy unsuccessful!</h1>';
    $errors=1;
}}}}

 if(isset($_POST['Submit']) && !$errors) 
 {
    echo "<h1>File Uploaded Successfully! Try again!</h1>";
 }

可能的问题和解决方案是什么