My solution for cache busting in WordPress (drop-in to `/mu-plugins/` in Bedrock).

version-string.php

<?php

/**
 * Replaces query version in registered scripts or styles with file modified time
 *
 * @link https://wordpress.stackexchange.com/a/346024/51130 based on
 *
 * @param $src
 *
 * @return string
 */
function add_modified_time( $src ) {
    $clean_src = remove_query_arg( 'ver', $src );
    $path      = wp_parse_url( $src, PHP_URL_PATH );
    $filename  = untrailingslashit( ABSPATH ) . '/..' . $path;
    if ( $modified_time = @filemtime( $filename ) ) {
        $src = add_query_arg( 'ver', $modified_time, $clean_src );
    } else {
        // $src = add_query_arg( 'ver', time(), $clean_src );
    }

    return $src;

}

add_filter( 'style_loader_src', 'add_modified_time', 99999999, 1 );
add_filter( 'script_loader_src', 'add_modified_time', 99999999, 1 );