nested-loop-9603-wordpress-final.php

 <?php 
    $par_args = array(
        'parent' => 0
    );

    $parent_categories = get_categories($par_args); 

    foreach($parent_categories as $parent_category) {
        echo '<h2 class="ui header">' . $parent_category->name . '</h2>'; 

        $chi_args = array(
            'child_of' => $parent_category->cat_ID
        );

        $children_categories = get_categories($chi_args);

        foreach($children_categories as $children_category) {
            echo '<li><a href="'. get_category_link($children_category->cat_ID) .'">' . $children_category->name . '</a></li>'; 
        }

    }
?>