An example of integration Slim Framework into 1C-Bitrix.

bootstrap.php

<?php
# local/php_interface/bootstrap.php

declare(strict_types = 1);

define('NOT_CHECK_PERMISSIONS', true);
define('NO_AGENT_CHECK', true);

$_SERVER['DOCUMENT_ROOT'] = dirname(__DIR__, 2);

require $_SERVER['DOCUMENT_ROOT'] . '/bitrix/modules/main/include/prolog_before.php';

use Bitrix\Main\Application;

/**
 * Урезанная инициализация 1C-Bitrix.
 *
 * @throws \Bitrix\Main\SystemException
 */
function initBitrixCore()
{
	global $DB;

	$DB->db_Conn = Application::getInstance()::getConnection()->getResource();

	// Авторизация под администратором
	$_SESSION['SESS_AUTH']['USER_ID'] = 1;
}

index.php

<?php
# api/index.php

declare(strict_types = 1);

define('NOT_CHECK_PERMISSIONS', true);

require_once $_SERVER['DOCUMENT_ROOT'] . '/local/php_interface/bootstrap.php';

use Slim\App as Slim;
use Slim\Container;
use Slim\Http\Request;
use Slim\Http\Response;

$container = new Container([
    'settings' => [
        'addContentLengthHeader' => true,
        'debug'                  => true,
        'displayErrorDetails'    => true,
        'routerCacheFile'        => false,
    ],
]);

/**
 * Переписанный обработчик исключений.
 * Все ошибки генерируются в JSON формате.
 *
 * @return \Closure
 */
$container['errorHandler'] = function () {
    return function (Request $request, Response $response, Throwable $exception) {
        return $response->withJson([
            'file'    => $exception->getFile(),
            'message' => $exception->getMessage(),
            'trace'   => $exception->getTrace(),
        ], 500);
    };
};

$app = new Slim($container);

$app->group('/api', function (Slim $app) use ($key) {
    $app->get('/test', function (Request $request, Response $response) {
        return $response->withJson(['msg' => 'Hello, World!']);
    });
});

$app->run();

urlrewrite.php

<?php
# urlrewrite.php

declare(strict_types = 1);

$arUrlRewrite = [
	0 => [
		'CONDITION' => '#^/rest/#',
		'RULE'      => '',
		'ID'        => null,
		'PATH'      => '/bitrix/services/rest/index.php',
		'SORT'      => 100,
	],
	1 => [
		'CONDITION' => '#^/api/#',
		'RULE'      => '',
		'ID'        => null,
		'PATH'      => '/api/index.php',
		'SORT'      => 100,
	],
];