determining file extension

[pit_of_zombies]

i'm having a hell of a time figuring out how to make this script works so that it doesn't upload when the file extension isn't .txt . Please help!

sorry i messed up in this post look at the code below.

[pit_of_zombies]

<?php

$uploaddir = '/usr/sites/stupidgamers.com/txt/';
$uploadfile = $uploaddir. $_FILES['userfile']['name'];

print "<pre>";

if(substr($_FILES['userfile']['tmp_name'], -3) == "txt") {

if (move_uploaded_file($_FILES['userfile']['tmp_name'], $uploadfile)) {
} else {
print "Wrong File Type";
}
print "File Uploaded.";

} else {

print "Cannot Upload.";
}


print "</pre>";



?>