Posts by categories

<?php
    /** Start copy/paste */
    /**
     * this function does all the work:
     * asks for a list of categories
     * if that is successful,
     * retrieves the posts associated with
     * each category
     */
    function list_posts_by_cat() {
        $taxonomy = 'category';
        $param_type = 'category__in';
        $term_args = array(
            'orderby' => 'name',
            'order' => 'ASC'
        );
        $terms = get_terms( $taxonomy, $term_args );

        $list = '';
        if ( $terms ) {
            foreach( $terms as $term ) {
                $qargs = array(
                    "$param_type" => array( $term->term_id ),
                    'post_type' => 'post',
                    'post_status' => 'publish',
                    'posts_per_page' => -1,
                    'caller_get_posts'=> 1
                );
                $new_posts = null;
                $new_posts = new WP_Query( $qargs );

                if( $new_posts->have_posts() ) {
                    $list .= "<div class='post-list-by-cat'>" . PHP_EOL;
                    $list .= "\t<h4>List of Posts in $taxonomy $term->name</h4>" . PHP_EOL;
                    $list .= "\t<ul>" . PHP_EOL;
                    while ( $new_posts->have_posts() ) :
                        $new_posts->the_post();
                        $list .= "\t\t<li><a href='" . get_permalink() . "' rel='bookmark' title='Permanent Link to " . the_title_attribute( 'echo=0' ) . "'>" . get_the_title() . "</a></li>" . PHP_EOL;
                    endwhile;
                    $list .= "\t</ul>" . PHP_EOL;
                    $list .= "</div>" . PHP_EOL;
                }
            }
            return $list;
        }
        wp_reset_query();
    }
    add_shortcode( 'listposts', 'list_posts_by_cat' );
    /** END copy/paste */
?>