I had the same problem.
The problem is that in authorize.php starting on line 53, it is listed
<form method="post" action="authorize.php">
<?php
print_r($auth_params);
foreach ($auth_params as $key => $value) : ?>
<input type="hidden"
name="<?php htmlspecialchars($key, ENT_QUOTES); ?>"
value="<?php echo htmlspecialchars($value, ENT_QUOTES); ?>" />
<?php endforeach; ?>
Do you authorize the app to do its thing?
<p><input type="submit" name="accept" value="Yep" /> <input
type="submit" name="accept" value="Nope" /></p>
</form>
And
name="<?php htmlspecialchars($key, ENT_QUOTES); ?>"
is missing echo, so it should be:
name="<?php echo htmlspecialchars($key, ENT_QUOTES); ?>"
and then the values do get passed and it works out.