Demonstration of the Register Helper "depends" field option.

my_pmprorh_depends_fields_init.php

<?php
function my_pmprorh_depends_fields_init()
{
    //don't break if Register Helper is not loaded
    if(!function_exists("pmprorh_add_registration_field"))
    {
	return false;
    }

    //add a new "About Your Pets" box on checkout form
    pmprorh_add_checkout_box("pets", "About Your Pet");

    //define the fields
    $fields = array();
    $fields[] = new PMProRH_Field(
    	"pet",
    	"select",
    	array(
		"label"=>"I have a:",
		"options"=>array(
			"" => "Select",
			"cat"=>"Cat",
			"dog"=>"Dog"
        )));
    $fields[] = new PMProRH_Field(
        "cat_name",
        "text",
        array(
		"depends"=>array(array('id' => "pet", 'value' => "cat")),
		"label"=>"Cat's Name",
        ));
	$fields[] = new PMProRH_Field(
        "dog_name",
        "text",
        array(
		"depends"=>array(array('id' => "pet", 'value' => "dog")),
		"label"=>"Dog's Name",
        ));
	
    //add the fields into a new "pets" area of the checkout page
    foreach($fields as $field)
        pmprorh_add_registration_field(
            "pets",
            $field
        );
}
add_action("init", "my_pmprorh_depends_fields_init");

This recipe is included in the blog post on "Exploring Register Helper: Adding Conditional Fields to Member Profile and Checkout" at Paid Memberships Pro here: https://www.paidmembershipspro.com/exploring-register-helper-adding-conditional-fields-member-profile-checkout/