Set timestamp to Wordpress child theme custom CSS file

timestamp.php

<?php
function my_child_theme_enqueue_styles() {
    $css_file = get_stylesheet_directory() . '/css/style.css';
    $css_url  = get_stylesheet_directory_uri() . '/css/style.css';

    wp_enqueue_style(
        'child-style',
        $css_url,
        array(), // No dependencies
        file_exists($css_file) ? filemtime($css_file) : null // Adds timestamp version
    );
}
add_action('wp_enqueue_scripts', 'my_child_theme_enqueue_styles');