php simple proxy

<?php

namespace main;

define("BACKEND_SOCKET", '<where the unix socket>');
define("BASE_URL", '<where this file located>');

function exit_500($msg) {
  $msg = rtrim($msg, "\n") . "\nDebug trace:\n";
  foreach(debug_backtrace() as $trace) {
    if(isset($trace['file']) && isset($trace['line'])) {
      $msg .= '- ' . $trace['file'] . ':' . $trace['line'] . "\n";
    }
  }
  error_log(substr($msg, 0, 2048));
  if(!headers_sent()) {
    http_response_code(500);
    header('Content-Type: text/plain');
    $level = ob_get_level();
    for($i = 0; $i < $level; $i++) ob_end_clean();
    echo "500 Internal Server error\n\n$msg";
  }
  exit();
}

set_error_handler(function($no, $msg) { exit_500("error: level errno $no: msg: $msg"); });
set_exception_handler(function($exception) { exit_500("exception: $exception"); });
ini_set('display_errors', 0);
error_reporting(0);

$stat = stat(BACKEND_SOCKET);
if(($stat !== FALSE) && (!(($stat['mode'] & 0170000) == 0140000))) {
  exit_500("gateway error: " . BACKEND_SOCKET . " is not a unix socket");
}

$method = strtoupper($_SERVER['REQUEST_METHOD']);

$uri = isset($_SERVER['PATH_INFO']) ? $_SERVER['PATH_INFO'] : "/";
if(isset($_SERVER['QUERY_STRING']) && strlen($_SERVER['QUERY_STRING']) > 0) {
  $uri .= "?{$_SERVER['QUERY_STRING']}";
}

$header_in = apache_request_headers();
$header_tobe_removed = array(
  'connection', 'keep-alive', 'proxy-authenticate', 'proxy-authorization', 'proxy-connection'
  'te', 'trailer', 'transfer-encoding', 'upgrade',
  'host', 'x-real-ip', 'x-forwarded-for'
);
foreach($header_in as $k => $v) {
  if(strtolower($k) == 'connection') {
    foreach(explode(",", $v) as $h) array_push($header_tobe_removed, strtolower(trim($h)));
  }
}

$header_out = array();
foreach($header_in as $k => $v) {
  if(!in_array(strtolower($k), $header_tobe_removed)) {
    array_push($header_out, "$k: $v");
  };
}
array_push($header_out, "Host: " . 'service');
array_push($header_out, "X-Real-IP: " . $_SERVER['REMOTE_ADDR']);
array_push($header_out, "X-Forwarded-For: " . $_SERVER['REMOTE_ADDR']);
array_push($header_out, "X-Base-Url: " . BASE_URL);

$stdin = fopen('php://input', 'rb');
$stdout = fopen('php://output', 'wb');
$curl = curl_init();

curl_setopt($curl, CURLOPT_UNIX_SOCKET_PATH,  BACKEND_SOCKET);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST,     $method);
curl_setopt($curl, CURLOPT_URL,               "http://service$uri");
curl_setopt($curl, CURLOPT_HTTPHEADER,        $header_out);
if (
  $method == "POST"                           ||
  $method == "PUT"                            ||
  $method == "PATCH"                          ||
  $method == "OPTIONS"                        ||
  isset($_SERVER['HTTP_TRANSFER_ENCODING'])   ||
  isset($_SERVER['CONTENT_LENGTH'])
) {
  curl_setopt($curl, CURLOPT_UPLOAD,          TRUE);
  curl_setopt($curl, CURLOPT_INFILE,          $stdin);
}
curl_setopt($curl, CURLOPT_HEADERFUNCTION,    function($curl, $data) {
  // TODO(win-t): remove hop-by-hop response header
  header($data);
  return strlen($data);
});
curl_setopt($curl, CURLOPT_FILE,              $stdout);

$curl_result = curl_exec($curl);
$curl_errno  = curl_errno($curl);
if(($curl_result === FALSE) || ($curl_errno != 0)) {
  $errstr = curl_strerror($curl_errno);
  exit_500("curl error: $errstr");
}

curl_close($curl);
fclose($stdout);
fclose($stdin);

// TODO(win-t): handle response 101 Switching Protocol