why can't upload image

<?php

$allowed_filetypes = array('.jpg','.gif','.bmp','.png');

$max_filesize = 5242888;

$upload_path = '/files';

$filename =$_FILES['userfile']['name'];

$ext = substr($filename,strpos($filename,'.'),strlen($filename)-1);//get the extension form the filename

if(!in_array($ext,$allowed_filetypes))
die('the file you attempted to upload is not allowed.');

if(filesize($_FILES['userfile']['size'])>$max_filesize)
die('the file you attempted to upload is too large.');


if(!is_writable($upload_path)){
die('you cannot upload to the specified directory,please CHMOD it to 777.');
}


if(move_uploaded_file($_FILES['userfile']['tmp_name'],$upload_path.$filename)){
echo 'you file upload successful.view the file <a href=".$upload_path.$filename.title="your file">here</a>';
}
else{
echo 'failed';
}



the code is the upload.php,what's wrong with it. it can't upload a jpg image.

the main html


<form action="upload.php" method="post" enctype="multipart/form-data">
<input type="file" name="userfile" id="file"/>
<button>upload</button>

<form action="upload.php" method="post">

You need to specify the enctype attribute of the form, with the required value, read http://htmldog.com/reference/htmltags/form/

You need to specify the enctype attribute of the form, with the required value, read http://htmldog.com/reference/htmltags/form/

i have changed it to "<form action="upload.php" method="post" enctype="multipart/form-data">"
but it still can't work.

Add echo '<pre>';
print_r($_FILES);
echo '</pre>';
at the top and check the output.