Order Wordpress posts by multiple meta key values

WP-order-by-multiple-meta-keys.php

<?php

/**
 * Wordpress order by multiple meta key values
 *
 * @author Rene Korss <rene.korss@gmail.com>
 */

// Override orderby
function orderby_multiple_meta_keys( $orderby ){
    return str_replace( 'meta_value', 'meta_value, mt1.meta_value', $orderby );
}

$args = array(
    'meta_query' => array(
        array(
            'key'     => 'first_meta_key',
            'compare' => 'EXISTS'
        ),
        array(
            'key'     => 'second_meta_key',
            'compare' => 'EXISTS'
        )
    ),
    'orderby'   => 'meta_value',
    'order'     => 'ASC'
);

// Replace orderby
add_filter( 'posts_orderby', 'orderby_multiple_meta_keys' );

// Query
$the_query = new WP_Query( $args );

// Remove our orderby
remove_filter( 'posts_orderby', 'orderby_multiple_meta_keys' );