send an email with attachment using mailgun api

mailgun.php

<?php
define('MAILGUN_URL', 'https://api.mailgun.net/v3/{{DOMAIN}}');
define('MAILGUN_KEY', '{{APIKEY}'); 

/**
 * @param $to         - a@xyz.com or a@xyz.com, b@xyz.com
 * @param $from       - doe <doe@xyz.com>
 * @param $subject    - subject
 * @param $html       - rich body
 * @param $text       - text body
 * @param $reply_to   - doe <doe@xyz.com>
 * @param $bcc        - doe <doe@xyz.com>
 * @param $attachments- array of attachment
 */
function send($to, $from, $subject, $html, $text = null, $reply_to = null, $bcc = null, $attachments = [])
{

    if (is_array($to)) {
        $to = implode(',', $to);
    } else {
        $to = $to;
    }

    $array_data = [
        'from' => $from,
        'to' => $to,
        'subject' => $subject,
        'html' => $html,
        'text' => $text,
        'o:tracking' => 'yes',
        'o:tracking-clicks' => 'yes',
        'o:tracking-opens' => 'yes',
    ];

    if ($reply_to) {
        $array_data['h:Reply-To'] = $reply_to;
    }

    if ($bcc) {
        $array_data['bcc'] = $bcc;
    }

    $index = 1;
    foreach ($attachments as $attachment) {
        if (file_exists($attachment)) {
            $array_data['attachment[' . $index . ']'] = curl_file_create($attachment, mime_content_type($attachment), basename($attachment));
            $index++;
        }
    }

    $ch = curl_init(MAILGUN_URL . '/messages');
    curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
    curl_setopt($ch, CURLOPT_USERPWD, 'api:' . MAILGUN_KEY);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $array_data);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: multipart/form-data'));
    curl_setopt($ch, CURLOPT_ENCODING, 'UTF-8');
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, false);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    $response = curl_exec($ch);
    curl_close($ch);
    $results = json_decode($response, true);
    return $results;
}