Enviar email usando la libreria PHPMailer

send_mail.php

<?php

/**
 * Envio de emails.
 * 
 * @param string $to Destinatario del email.
 * @param string $subject Asunto.
 * @param string $body Mensaje.
 * @param string $alt_body (Opcional) Mensaje en texto plano.
 * @return mixed Devuelve TRUE si todo paso bien o un String con el error si algo fallo.
 */
function send_mail($to, $subject, $body, $alt_body = '', $attachments = array())
{
    require_once APP_PATH . '/lib/PHPMailer_v5.1/class.phpmailer.php';
    require_once APP_PATH . '/lib/PHPMailer_v5.1/class.smtp.php';

    $mail = new PHPMailer(TRUE);
    //$mail->SMTPDebug  = 2;
    
    
    $mail->IsSMTP();
    $mail->SMTPAuth = SMTP_AUTH;
    $mail->SMTPSecure = SMTP_SECURE;
    $mail->Port = SMTP_PORT;
    $mail->Host = SMTP_HOST;
    $mail->Username = SMTP_USER;
    $mail->Password = SMTP_PASS;

    $mail->SetFrom(SMTP_FROM, SMTP_FROM_NAME);
    $mail->AddReplyTo(SMTP_FROM, SMTP_FROM_NAME);

    // Se setea el asunto y el mensaje.
    $mail->Subject = $subject;
    
    if (strlen($alt_body) > 0)
        $mail->AltBody = $alt_body;
    
    $mail->MsgHTML($body);
    
    // Archivos adjuntos.
    if (count($attachments) > 0)
    {
        foreach ($attachments as $attachment)
        {
            if (!is_file($attachment)) continue;
            
            $mail->AddAttachment($attachment, basename($attachment), 'base64', mime_content_type($attachment));
        }
    }
    
    if (!$mail->ValidateAddress($to)) return 'Invalid email address.';

    // Se setea el destinatario.
    if (is_array($to) && count($to) === 2)
        $mail->AddAddress($to[0], $to[1]);
    else if (is_array($to) && count($to) === 1)
        $mail->AddAddress($to[0]);
    else if (is_string($to))
        $mail->AddAddress($to);

    // Se intenta enviar.
    try {
        $result = $mail->Send();
    } catch (Exception $exc) {
        $result = $exc->getMessage();
    }
    
    // Se resetean los destinatarios.
    $mail->ClearAddresses();
    
    //---------------------------------------------------
    return $result;
}