WordPress - Sending multipart (text/html and plain/text) email in WordPress programatically.

wp.function.multipart.mail.php

/**
 * Sends a email in both 'text/html' and 'plain/text'.
 * 
 * @param string|array $to
 * @param string $subject
 * @param string $html
 * @param string $text
 * @return boolean|string
 * @see stackoverflow.com/questions/22507176
 */
function send_multipart_mail($to, $subject, $html, $text) {
	$result = false;
	
	global $phpmailer;
	
	// get PHPMailer if it was not constructed yet
	if(!($phpmailer instanceof PHPMailer)) {
		require_once ABSPATH . WPINC . '/class-phpmailer.php';
		require_once ABSPATH . WPINC . '/class-smtp.php';
		$phpmailer = new PHPMailer(true);
	}
	
	$mail =& $phpmailer;
	
	// Empty out the values that may be set
	$mail->ClearAllRecipients();
	$mail->ClearAttachments();
	$mail->ClearCustomHeaders();
	$mail->ClearReplyTos();
	
	// sets the default 'from' name
	$from_name = 'Website';	
	
	if($site_title = get_bloginfo('name')) {
		$from_name = $site_title;
	}

	// sets the default 'from' email address
	if(!isset($from_email)) {
		$domain = strtolower($_SERVER['SERVER_NAME']);
		
		if(substr($domain, 0, 4) == 'www.') {
			$domain = substr($domain, 4);
		}

		$from_email = 'info@' . $domain;
	}

	// filter from email address and name
	$mail->From = apply_filters('wp_mail_from', $from_email);
	$mail->FromName = apply_filters('wp_mail_from_name', $from_name);
	
	// indicate 
	$mail->IsHTML(true);
	
	$mail->CharSet = "text/html; charset=UTF-8;";
	$mail->WordWrap = 80;

	// Sender information
	$mail->From = $from_email;
	$mail->FromName = $from_name;
	
	// let plugins have their way before we do our thing
	do_action_ref_array('phpmailer_init', [&$mail]);
	
	// Recipient information
	foreach((array)$to as $to_name => $to_email) {
		if(is_numeric($to_name)) {
			$to_name = '';
		}
		
		$mail->AddAddress($to_email, $to_name);
	}
	
	// Content information
	$mail->Subject =  $subject;
	$mail->Body = $html; 
	
	// This automatically sets the email to multipart/alternative.
	// This body can be read by mail clients that do not have HTML
	$mail->AltBody = $text;
	
	// send email
	if($mail->Send()) {
		$result = true;
	}
	else if($mail->ErrorInfo) {
		$result = $mail->ErrorInfo;
	}
	
	return $result;
}