Here’s a code snippet on how to send HTML email with SMTP authentication using PHP. You would have to download a library called PHPMailer as the mail() function of PHP does not support SMTP authentication. There is also other class that must be in the same location as class.phpmailer.php in order for the mail sending to work, smtp.phpmailer.php.

require_once("class.phpmailer.php");
$mail = new PHPMailer();
$mail->IsSMTP(); // telling the class to use SMTP
$mail->IsHTML(true);
$mail->SMTPAuth = true;
$mail->Username = "username"; // your SMTP username
$mail->Password = "password"; // your SMTP password
$mail->Host = "smtp.host.url"; // SMTP server
$mail->From = $from;
$mail->AddAddress($to);

$mail->Subject = $subject;
$mail->Body = $body;

if(!$mail->Send()) {
  echo 'Message was not sent.';
  echo 'Mailer error: ' . $mail->ErrorInfo;
} else {
  echo 'Message has been sent.';
}