Since symfony 4.4 the symfony/debug component is deprecated.

The “\Symfony\Component\Debug\ErrorHandler” class is deprecated since Symfony 4.4, use “\Symfony\Component\ErrorHandler\ErrorHandler”

Running unit tests on Mascot, I noticed lots of warnings.

But it was not easy as changing Symfony\Component\Debug\ExceptionHandler to Symfony\Component\ErrorHandler\ErrorHandler.

Assume we have something as below

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<?php
$handler = new \Symfony\Component\Debug\ExceptionHandler($debug);
$exception = $event->getThrowable();
if (!$exception instanceof FlattenException) {
    $exception = FlattenException::create($exception);
}
$response = Response::create($handler->getHtml($exception), $exception->getStatusCode(), $exception->getHeaders())->setCharset(ini_get('default_charset'));
$event->setResponse($response);

The Symfony\Component\ErrorHandler\ErrorHandler has no getHtml method.

From the symfony docs, if you are in development mode we can use below code.

<?php
if ($_SERVER['APP_DEBUG']) {
    Debug::enable();
}

so I started with

<?php
ErrorHandler::register();

But it was throwing all details. I was struggling how to fix this. After many failed experiments, I asked Yonel Ceruto Gonzalez.

Sets the scope at 0 to tell the handler that it is in “non-debug” mode:

$errorHandler->scopeAt(0, true);

However, don't expect the real exception message in non-debug mode, it could reveal sensitive information.

— Yonel Ceruto Gonzalez¹

The above information was really helpful to do more experiments. I finally fixed the unit tests with the below code.

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<?php
$handler = new \Symfony\Component\ErrorHandler\ErrorHandler();
$handler->setExceptionHandler([$handler, 'renderException']);
if (! $this->debug) {
    $handler->scopeAt(0, true);
}
$exception = $event->getThrowable();
ob_start();
$handler->handleException($exception);
$response = ob_get_clean();
if (!$exception instanceof FlattenException) {
    $exception = FlattenException::create($exception);
}
$response = Response::create($response, $exception->getStatusCode(), $exception->getHeaders())->setCharset(ini_get('default_charset'));

I hope this information may help someone at somepoint of time.