PHP fájlfeltöltés miért nem működik nekem?

HTML kód:

<input name="img" type="file">

PHP kód:

// UPLOAD PROFILE IMG

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

$max_filesize = 10485760;

$upload_path = 'userdata/profile_img/';

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

$ext = substr($filename, strpos($filename,'.'), strlen($filename)-1);

$filename = $data["id"].".png";

if(!in_array($ext,$allowed_filetypes))

die('The file you attempted to upload is not allowed.');

if(filesize($_FILES['img']['tmp_name']) > $max_filesize)

die('The file you attempted to upload is too large.');

if(move_uploaded_file($_FILES['img']['tmp_name'],$upload_path . $filename)) {

// Its seems okay

}

Kimenetel:

Undefined index: img on line 40

40. sor:

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

Mi a hiba? :(