Answer by andrewsi for Wordpress category query not working

You're almost there.

Have a look at the manual page for single_cat_title(), you'll see that it takes two parameters. The first is a prefix to use in the return; the second is the important one here - it defines whether you want the category to be echoed out (if true), or returned (if false). So all you need to do is swap:

$thecat = single_cat_title( '', true );

to:

$thecat = single_cat_title( '', false );

In your current code, you're just echoing the title, and $thecat is being set to null.