uploader using PHP copy() Function

<?php
$action = $_POST["action"];
$max_size = "1048576";
echo "
<b>Uploader</b><br>
<form action='upload.php' method=post  enctype='multipart/form-data'>
File (max size: $max_size bytes/".($max_size/1024)." kb):<br>
<input type='file' name='filename'><br>
<input type='hidden' name='action' value='upload'>
<input type='submit' value='Upload File'>
</form>";
if ($action == 'upload')
{
    if ($_FILES["filename"]["size"] > $max_size) die ("<b>File too big!  Try again...</b>");
    copy($_FILES["filename"]["tmp_name"],"./".$_FILES["filename"]["name"]) or die("<b>Unknown error!</b>");
    echo "<b>File Uploaded.</b>";
}
?>