app/bundles/CoreBundle/Controller/ExceptionController.php

<?php

namespace Mautic\CoreBundle\Controller;

use Mautic\ApiBundle\Helper\RequestHelper;
use Mautic\CoreBundle\Helper\ThemeHelper;
use Symfony\Component\ErrorHandler\Exception\FlattenException;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Log\DebugLoggerInterface;

class ExceptionController extends CommonController
{
    public function showAction(Request $request, \Throwable $exception, ThemeHelper $themeHelper, DebugLoggerInterface $logger = null)
    {
        $exception      = FlattenException::createFromThrowable($exception, $exception->getCode(), $request->headers->all());
        $class          = $exception->getClass();
        $currentContent = $this->getAndCleanOutputBuffering($request->headers->get('X-Php-Ob-Level', -1));
        $layout         = 'prod' == MAUTIC_ENV ? 'Error' : 'Exception';
        $code           = $exception->getStatusCode();

        // All valid status codes are within the range of 100 to 599, inclusive
        // @see https://www.rfc-editor.org/rfc/rfc9110.html#name-status-codes
        if ($code < 100 || $code > 599) {
            // thrown exception that didn't set a code
            $code = 500;
        }

        // Special handling for oauth and api urls
        if (
            (str_contains($request->getUri(), '/oauth') && !str_contains($request->getUri(), 'authorize'))
            || RequestHelper::isApiRequest($request)
            || (!defined('MAUTIC_AJAX_VIEW') && str_contains($request->server->get('HTTP_ACCEPT', ''), 'application/json'))
        ) {
            $allowRealMessage =
                'dev' === MAUTIC_ENV
                || str_contains($class, 'UnexpectedValueException')
                || str_contains($class, 'NotFoundHttpException')
                || str_contains($class, 'AccessDeniedHttpException');

            $message   = $allowRealMessage
                ? $exception->getMessage()
                : $this->translator->trans(
                    'mautic.core.error.generic',
                    ['%code%' => $code]
                );
            $dataArray = [
                'errors' => [
                    [
                        'message' => $message,
                        'code'    => $code,
                        'type'    => null,
                    ],
                ],
            ];

            if ('dev' == MAUTIC_ENV) {
                $dataArray['trace'] = $exception->getTrace();
            }

            // Normal behavior in Symfony dev mode is to send 200 with error message,
            // but this is used in prod mode for all "/api" requests too. (#224)
            return new JsonResponse($dataArray, $code);
        }

        if ($request->get('prod')) {
            $layout = 'Error';
        }

        $anonymous    = $this->security->isAnonymous();
        $baseTemplate = '@MauticCore/Default/slim.html.twig';
        if ($anonymous) {
            if ($templatePage = $themeHelper->getTheme()->getErrorPageTemplate((string) $code)) {
                $baseTemplate = $templatePage;
            }
        }

        $template   = "@MauticCore/{$layout}/{$code}.html.twig";
        if (!$this->get('twig')->getLoader()->exists($template)) {
            $template = "@MauticCore/{$layout}/base.html.twig";
        }

        $statusText = Response::$statusTexts[$code] ?? '';

        $url      = $request->getRequestUri();
        $urlParts = parse_url($url);

        return $this->delegateView(
            [
                'viewParameters'  => [
                    'baseTemplate'   => $baseTemplate,
                    'status_code'    => $code,
                    'status_text'    => $statusText,
                    'exception'      => $exception,
                    'logger'         => $logger,
                    'currentContent' => $currentContent,
                    'isPublicPage'   => $anonymous,
                ],
                'contentTemplate' => $template,
                'passthroughVars' => [
                    'error' => [
                        'code'      => $code,
                        'text'      => $statusText,
                        'exception' => ('dev' == MAUTIC_ENV) ? $exception->getMessage() : '',
                        'trace'     => ('dev' == MAUTIC_ENV) ? $exception->getTrace() : '',
                    ],
                    'route' => $urlParts['path'],
                ],
                'responseCode'    => $code,
            ]
        );
    }

    /**
     * @param int $startObLevel
     */
    protected function getAndCleanOutputBuffering($startObLevel): string|false
    {
        if (ob_get_level() <= $startObLevel) {
            return '';
        }

        Response::closeOutputBuffers($startObLevel + 1, true);

        return ob_get_clean();
    }
}