functions.php

<?php
//* Don't include the opening php tag

/**
 * Add an 'envira' shortcode to all default WordPress galleries
 */
add_shortcode( 'gallery', 'prefix_envira_to_gallery_shortcode' );
function prefix_envira_to_gallery_shortcode( $atts ) {
    $atts[ 'envira'] = 'true';
    return gallery_shortcode( $atts );
}