Send mail with mailgun api by PHP CURL.

function.php

<?php 
define('MAILGUN_URL', 'https://api.mailgun.net/v3/DOMAIN_NAME');
define('MAILGUN_KEY', 'KEY'); 
function sendmailbymailgun($to,$toname,$mailfromname,$mailfrom,$subject,$html,$text,$tag,$replyto){
    $array_data = array(
		'from'=> $mailfromname .'<'.$mailfrom.'>',
		'to'=>$toname.'<'.$to.'>',
		'subject'=>$subject,
		'html'=>$html,
		'text'=>$text,
		'o:tracking'=>'yes',
		'o:tracking-clicks'=>'yes',
		'o:tracking-opens'=>'yes',
		'o:tag'=>$tag,
		'h:Reply-To'=>$replyto
    );
    $session = curl_init(MAILGUN_URL.'/messages');
    curl_setopt($session, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
  	curl_setopt($session, CURLOPT_USERPWD, 'api:'.MAILGUN_KEY);
    curl_setopt($session, CURLOPT_POST, true);
    curl_setopt($session, CURLOPT_POSTFIELDS, $array_data);
    curl_setopt($session, CURLOPT_HEADER, false);
    curl_setopt($session, CURLOPT_ENCODING, 'UTF-8');
    curl_setopt($session, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($session, CURLOPT_SSL_VERIFYPEER, false);
    $response = curl_exec($session);
    curl_close($session);
    $results = json_decode($response, true);
    return $results;
}