Woocommerce Subscriptions - Increase the subscription period interval (e.g. Lifetime subscriptions)

web357_woocommerce_subscription_lifetime.php

<?php
/**
 * If you would like to increase the subscription period interval (e.g. for lifetime subscriptions).
 */
add_action( 'admin_enqueue_scripts', 'web357_woocommerce_subscription_lifetime' );
function web357_woocommerce_subscription_lifetime() 
{
    global $post;

    if ( ! wp_script_is( 'jquery', 'done' ) ) {
      wp_enqueue_script( 'jquery' );
    }

    $subscription_period_interval = get_post_meta( $post->ID, '_subscription_period_interval', true );
    if ($subscription_period_interval > 0)
    {
        $js_code = <<<JS
        jQuery(document).ready(function($){
            var spi_selected = {$subscription_period_interval} === 100 ? ' selected' : '';
            $('#_subscription_period_interval').append('<option value="100"'+spi_selected+'>every 100th</option>'); 
        });		
JS;
    }

    wp_add_inline_script( 'jquery-migrate', $js_code );
 }
?>