Need help category title in single post template

When I use the function single_cat_title() inside the single.php template, it returns a null value. I’m using this to include a php code relative to post category.

It’s like this:

<?php
$categoria = single_cat_title(”,false);
$categoria_out = dirify($categoria);
?>

This code work perfectly in category template, but in single post template I have this problem – single_cat_title() returns a null value.

How I can display the category name in single.php template outside The Loop?