Wordpress list all posts by categories

gistfile1.aw

<?php
foreach( get_categories('hide_empty=1','order_by=name') as $cat ) :
	if( !$cat->parent ) {
		echo '<h2><a href="#">' . $cat->name . '</a></h2>';
		echo '<ul>';
		process_cat_tree( $cat->term_id );
	}
	endforeach;
	wp_reset_query(); //to reset all trouble done to the original query

function process_cat_tree( $cat ) {
	$args = array('category__in' => array( $cat ), 'numberposts' => -1, 'orderby' => title, 'order' => ASC);
	$cat_posts = get_posts( $args );
	global $post;
	$IDOutsideLoop = $post->ID;
	
	if( $cat_posts ) : 
	foreach( $cat_posts as $post ) : ?>
		<li <?php if(is_single() && $IDOutsideLoop == $post->ID) print 'class="current-item"'; ?>><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
 	<?php endforeach;
 	endif;
 
	$next = get_categories('hide_empty=0&parent=' . $cat);
 
	if( $next ) : 
	foreach( $next as $cat ) :
		echo '<ul><li><strong>' . $cat->name . '</strong></li>';
 		process_cat_tree( $cat->term_id );
 	endforeach;
	endif;
 
	echo '</ul>';
}
?>