Blog filter

<form action="<?php echo site_url() ?>/wp-admin/admin-ajax.php" method="POST" id="filter">

    <?php

        if( $terms = get_terms( array( 'taxonomy' => 'category', 'orderby' => 'name' ) ) ) : 

            echo '<select name="categoryfilter"><option>Select category...</option>';

            foreach ( $terms as $term ) :

                echo '<option value="' . $term->term_id . '">' . $term->name . '</option>'; 

            endforeach;

            echo '</select>';

        endif;

    ?>

    <input type="hidden" name="action" value="myfilter">

    <button>Apply filter</button>

</form>