How to Dynamically send a newsletter with MailChimp 3.0 via PHP

mailchimp_3.0.php

<?php

/**
 * Reference Links:
 * - https://mailchimp.com/help/find-your-list-id/
 * - https://stackoverflow.com/a/48655086/5228251
 * - https://thedebuggers.com/send-mailchimp-newsletter-via-php/
 */

// Include Mailchimp API 3.0
include_once('./mailchimp.php'); // copy from here: https://github.com/drewm/mailchimp-api/blob/master/src/MailChimp.php

use \DrewM\MailChimp\MailChimp;

/* SET THE VARIABLES HERE BELOW */
// Configure --------------------------------------
$api_key = "YOUR_API_KEY"; // see: https://mailchimp.com/help/about-api-keys/
$list_id = "your list id here"; // see: https://mailchimp.com/help/find-your-list-id/
$subject = "Hello, You've successfully subscribed to our Newsletter!";
$reply_to = "Your Company Email Address"; 
$from_name = "Your Company Name";

$template_id = 99999; // INTEGER
$template_html = '<p>Ayo whatup? THIS IS THE SAMPLE CONTENT BODY OF MY EMAIL MESSAGE.</p>';
// STOP Configuring -------------------------------

// Start create email campaign ->
$MailChimp = new MailChimp($api_key);

// Create or Post new Campaign
$result = $MailChimp->post("campaigns", [
    'type' => 'regular',
    'recipients' => [
        'list_id' => $list_id
    ],
    'settings' => [
        'subject_line' => $subject,
        'reply_to' => $reply_to,
        'from_name' => $from_name
    ]
]);

$response = $MailChimp->getLastResponse();
$responseObj = json_decode($response['body']);

$campaign_id = $responseObj->id;

// Update Template content
// FYI: Make sure your template is labeled as "Code your own"
$result = $MailChimp->put('campaigns/' . $campaign_id . '/content', [
	'template' => array(
        'id' => (int)$template_id, 
        'sections' => array(
            'html' => $template_html
        )
    )
]);

// Send Campaign
$result = $MailChimp->post('campaigns/' . $campaign_id . '/actions/send');

?>