WordPress Get category slug using category ID

First, put the following function in your functions.php file:

function get_cat_slug($cat_id) {
 $cat_id = (int) $cat_id;
 $category = &get_category($cat_id);
 return $category->slug;
}

Once done, you can call the function as shown below:

<?php echo get_cat_slug(21); ?>

This will display the slug for the category with the ID 21.