Register Custom Query Vars

<?php
// register custom query args to allow use of get_query_var in search template
add_action( 'init', 'add_get_val' );
function add_get_val() {
     global $wp;
     $wp->add_query_var( '_categories' );
     $wp->add_query_var( '_post_type' );
     $wp->add_query_var( '_author' );
}