Linking Mailchimp to Sign-Up Boxes - php

Question

I am linking an e-mail sign-up and a post code (UK English for zip code) input box (see html code below) to an existing Mailchimp e-mail list. I have taken out the $apiKey, $listId and $submit_url identifiers from the code below. I have been able to successfully link the Mailchimp list to the e-mail input (so that e-mail address are populated when they are input in the Mailchimp list but I cannot figure out how to get post codes to automatically update in mailchimp list when they are input).
I am relatively new to php and would be so grateful for any help.
<div class="subscribe">
<form method="post" action="bokacsubscribe.php" role="form">
<input type="email" placeholder="Enter your e-mail address..." class="subscribe-input" name="email" style="margin-bottom: 10px;">
<form method="post" action="bokacsubscribe.php" role="form">
<input type="email" placeholder="Enter your postcode" class="subscribe-input" name="postcode" style="margin-bottom: 10px;">
<button type="submit" class="btn btn-subscribe success" style="border-radius:120px;background-color:rgb(241,174,200);border:1px solid rgb(241,174,200)"><b>Send</b></button>
<input type="hidden" name="referr" id="referr"/>
</form>
</div>
</div>
<?php
// Credits: https://gist.github.com/mfkp/1488819
session_cache_limiter('nocache');
$apiKey = ''; - // How get your Mailchimp API KEY - http://kb.mailchimp.com/article/where-can-i-find-my-api-key
$listId = ''; - // How to get your Mailchimp LIST ID - http://kb.mailchimp.com/article/how-can-i-find-my-list-id
$submit_url = ""; - // Replace us2 with your actual datacenter
$double_optin = false;
$send_welcome = false;
$email_type = 'html';
$email = $_POST['email'];
$merge_vars = array( 'YNAME' => $_POST['yname'] );
$data = array(
'email_address' => $email,
'apikey' => $apiKey,
'id' => $listId,
'double_optin' => $double_optin,
'send_welcome' => $send_welcome,
'merge_vars' => $merge_vars,
'email_type' => $email_type
);
$payload = json_encode($data);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $submit_url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, urlencode($payload));
$result = curl_exec($ch);
curl_close ($ch);
$data = json_decode($result);
if ($data->error) {
// do whatever you do on failure here
} else {
// do whatever you do on success here
$arrResult = array ('response'=>'success');
}