A simple but fast and functional PHP reverse proxy.

index.php

<?php

/* Copyright (C) 2019-2021, Manuel Meitinger
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */


//
// INIT
//

// be strict and define the proper end of line for HTTP
error_reporting(E_ALL);
define('EOL', "\r\n");

// disable any default charset (otherwise always gets appended)
ini_set('default_charset', '');

// compute the actual url
$host = 'example.org';
$ip = '1.2.3.4';
$port = 443;
$url = "https://$host:$port".$_SERVER['REQUEST_URI'];


//
// HELPERS
//

function buildPostDataForFields($delimiter, $name, $content)
{
  // handle array content specially
  if (is_array($content))
  {
    $data = '';
    foreach ($content as $key => $value)
    {
      $data .= buildPostDataForFields($delimiter, $name . '[' . $key . ']', $value);
    }
    return $data;
  }

  // return the field post data section
  return '--' . $delimiter . EOL
         . 'Content-Disposition: form-data; name="' . $name . '"' . EOL
         . EOL
         . $content . EOL;
}

function buildPostDataForFiles($delimiter, $name, $error, $fileName, $fileType, $filePath)
{
  // handle array content specially
  if (is_array($error))
  {
    $data = '';
    foreach ($error as $key => $value)
    {
      $data .= buildPostDataForFiles($delimiter, $name . '[' . $key . ']', $value, $fileName[$key], $fileType[$key], $filePath[$key]);
    }
    return $data;
  }

  // ensure the file was uploaded properly
  if ($error != UPLOAD_ERR_OK)
  {
    return '';
  }

  // create the file's post data section
  return '--' . $delimiter . EOL
         . 'Content-Disposition: form-data; name="' . $name . '"; filename="' . $fileName . '"' . EOL
         . 'Content-Transfer-Encoding: ' . $fileType . EOL
         . EOL
         . file_get_contents($filePath) . EOL;
}

function handleReturnHeader($curl, $header)
{
  // either set the response code or return the header
  if (preg_match('/^HTTP\/\d+\.\d+ (\d{3})/', $header, $code))
  {
    http_response_code($code[1]);
  }
  else
  {
    header($header);
  }
  return strlen($header);
}


//
// MAIN
//

// initialize cURL
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_RESOLVE,        array("$host:$port:$ip"));
curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, FALSE);
curl_setopt($curl, CURLOPT_FOLLOWLOCATION, FALSE);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST,  $_SERVER['REQUEST_METHOD']);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, FALSE);
curl_setopt($curl, CURLOPT_HEADERFUNCTION, 'handleReturnHeader');

// build the headers
$headers = array();
foreach($_SERVER as $key => $value)
{
  // forward all HTTP headers except the content length, which may change
  if (preg_match('/^HTTP_/', $key) && $key != 'HTTP_CONTENT_LENGTH')
  {
    // transform the key names to proper HTTP headers
    array_push($headers, str_replace(' ', '-', ucwords(strtolower(str_replace('_', ' ', substr($key, 5))))) . ': ' . $value);
  }
}

// append any post data
if (!empty($_SERVER['CONTENT_TYPE']))
{
  if (preg_match('/^multipart\/form-data/', strtolower($_SERVER['CONTENT_TYPE'])))
  {
    // create a unique delimiter
    $delimiter = '-------------' . uniqid();

    // build the post data for fields and files
    $postData = '';
    foreach ($_POST as $name => $content)
    {
      $postData .= buildPostDataForFields($delimiter, $name, $content);
    }
    foreach ($_FILES as $name => $content)
    {
      $postData .= buildPostDataForFiles($delimiter, $name, $content['error'], $content['name'], $content['type'], $content['tmp_name']);
    }
    $postData .= '--' . $delimiter . '--' . EOL;

    // set the content type to multipart with the new delimiter
    array_push($headers, 'Content-Type: multipart/form-data; boundary=' . $delimiter);
  }
  else
  {
    // simply forward the content and its type
    $postData = file_get_contents('php://input');
    array_push($headers, 'Content-Type: ' . $_SERVER['CONTENT_TYPE']);
  }

  // instruct curl to post data if there is any
  if ($postData)
  {
    curl_setopt($curl, CURLOPT_POST, TRUE);
    curl_setopt($curl, CURLOPT_SAFE_UPLOAD, TRUE);
    curl_setopt($curl, CURLOPT_POSTFIELDS, $postData);
  }
}

// set the headers and execute the query
curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
curl_exec($curl);
curl_close($curl);

?>