We also have an official API wrapper for PHP, which is available on Bitbucket or via Packagist, which wraps the Mandrill API for you.
If your Mandrill API key is stored as an environment variable, here's a simple example of sending using a template, with some merge variables and metadata:
<?php
require 'Mandrill.php';
$mandrill = new Mandrill();
// If are not using environment variables to specific your API key, use:
// $mandrill = new Mandrill("YOUR_API_KEY")
$message = array(
'subject' => 'Test message',
'from_email' => 'you@yourdomain.com',
'html' => '<p>this is a test message with Mandrill\'s PHP wrapper!.</p>',
'to' => array(array('email' => 'recipient1@domain.com', 'name' => 'Recipient 1')),
'merge_vars' => array(array(
'rcpt' => 'recipient1@domain.com',
'vars' =>
array(
array(
'name' => 'FIRSTNAME',
'content' => 'Recipient 1 first name'),
array(
'name' => 'LASTNAME',
'content' => 'Last name')
))));
$template_name = 'Stationary';
$template_content = array(
array(
'name' => 'main',
'content' => 'Hi *|FIRSTNAME|* *|LASTNAME|*, thanks for signing up.'),
array(
'name' => 'footer',
'content' => 'Copyright 2012.')
);
print_r($mandrill->messages->sendTemplate($template_name, $template_content, $message));
?>