WordPress Content Filter

<?php
/*
Plugin Name: dp Filter
Plugin URI: http://wpsv.se
Description: Demonstration of filtering with query
Version: 1.0
Author: Kenth Hagström
Author URI: http://designpalatset.se
License: GPL
Text Domain: dp-filter
Domain Path: /_languages/
*/

function dp_secondary_category() {

    $labels = array(
        'name'                => _x( 'Category 2', 'taxonomy general name' ),
        'singular_name'       => _x( 'Category 2', 'taxonomy singular name' ),
        'search_items'        => __( 'Search Category 2' ),
        'all_items'           => __( 'All Category 2' ),
        'parent_item'         => __( 'Parent Category 2' ),
        'parent_item_colon'   => __( 'Parent Category 2:' ),
        'edit_item'           => __( 'Edit Category 2' ),
        'update_item'         => __( 'Update Category 2' ),
        'add_new_item'        => __( 'Add New Category 2' ),
        'new_item_name'       => __( 'New Category 2 Name' ),
        'menu_name'           => __( 'Category 2' )
    );

    $args = array(
        'hierarchical'        => true,
        'labels'              => $labels,
        'show_ui'             => true,
        'show_admin_column'   => true,
        'query_var'           => true,
        'rewrite'             => array( 'slug' => 'category2' )
    );

    register_taxonomy( 'category2', 'post', $args );
}

add_action( 'init','dp_secondary_category' );


function dp_filter() {

    global $query_string;

    if ( 'POST' == $_SERVER['REQUEST_METHOD'] && 'dp-do-filter' == $_POST['action'] ) {

        echo '<pre>';
        print_r($_POST);
        echo '</pre>';

        $args = wp_parse_args( $query_string );

        $cat = !empty( $_POST['cat'] ) ? $_POST['cat'] : array();
        $cat2 = !empty( $_POST['cat2'] ) ? $_POST['cat2'] : array();

        $paged = max( 1, get_query_var('paged') );

        $args['tax_query'] = array(
            'relation' => 'OR',
            array(
                'taxonomy' => 'category',
                'terms' => $cat,
                'field' => 'id',
            ),
            array(
                'taxonomy' => 'category2',
                'terms' => $cat2,
                'field' => 'id',
            ),
        );

        $filter = new WP_Query( $args );

        echo '<div style="background: #CCC;">';
        if( $filter->have_posts() ):

            echo '<ul>';
            while( $filter->have_posts() ) : $filter->the_post();
                echo '<li><a href="' . the_permalink() . '">' . the_title() . '</a></li>';
            endwhile;
            echo '</ul>';

        endif;
        wp_reset_query();
        echo '</div>';

    } else {
    ?>
    <form id="dp-filter" name="dp-filter" method="post" action="" class="dp-filter-form" enctype="multipart/form-data">
        <label for="dropdown_category"><?php _e( 'Category', 'dp-filter' ); ?>:</label>
        <?php
        $cat = wp_dropdown_categories( array( 'echo' => 0, 'taxonomy' => 'category', 'hide_empty' => 1 ) );
        $cat = str_replace( "name='cat' id=", "name='cat[]' id=", $cat );
        echo $cat;
        ?>

        <label for="dropdown_category2"><?php _e( 'Category2', 'dp-filter' ); ?>:</label>
        <?php
        $cat2 = wp_dropdown_categories( array( 'echo' => 0, 'taxonomy' => 'category2', 'hide_empty' => 1 ) );
        $cat2 = str_replace( "name='cat2' id=", "name='cat2[]' id=", $cat2 );
        echo $cat2;
        ?>

        <input type="submit" value="<?php _e( 'Filter', 'dp-filter' ); ?>" tabindex="40" id="submit" name="submit">
        <input type="hidden" name="action" value="dp-do-filter">

    </form>
    <?php
    }
}

add_shortcode( 'dp_filter', 'dp_filter' ); // [dp_filter]

?>