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<?php

// ..........................
// your functions.php content
// ..........................

// copy paste the following functions anywhere in your theme its functions.php file

function my_prefix_set_mailchimp_cookie($email, $merge_vars, $form_id, $result) {
	if($result == true) {
		setcookie("mc4wp_subscribed", $email, time() + 31556926, '/');
	}
}

add_action('mc4wp_after_subscribe', 'my_prefix_set_mailchimp_cookie', 10, 4);

function my_prefix_show_stb_box($matched, $box_id) {

	if(isset($_COOKIE['mc4wp_subscribed'])) {
		return false;
	}

	return $matched;
}

add_action('stb_show_box', 'my_prefix_show_stb_box', 10, 2);