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Customizing Error Pages and How Errors are Handled

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Customizing Error Pages and How Errors are Handled

Sometimes things fall apart. And when they do, we show our users an error page. Hopefully, a hilarious error page.

Right now, our 404 page isn’t very hilarous, except for the little Pacman ghost that’s screaming “Exception detected”. He’s adorable.

We see this big descriptive error page because we’re in the dev environment, and Symfony wants to help us fix our mistake. In real life, also known as the prod environment, it’s different.

The Real Life: prod Environment

To see our app in its “real life” form, put an app.php after localhost:

We talked about environments and this app.php stuff in episode 1. If you don’t remember it, go back and check it out!

The page might work or it might be broken. That’s because we always need to clear our Symfony cache when going into the prod environment:

php app/console cache:clear --env=prod

Ok, now the site works. Invent a URL to see the 404 page. Ah gross! This error page isn’t hilarous at all! So where is the content for this page actually coming from and how can we make a better experience for our users?

Overriding the Error Template Content

To find out, let’s just search the project! In PHPStorm, I can navigate to vendor/symfony/symfony, right click, then select “Find in Path”. Let’s look for the “An Error Occurred” text.

Ah hah! It points us straight to a file in the core Twig bundle called error.html.twig. Let’s open that up!


The location of the file is:


Cool, so how can we replace this with a template that has unicorns, or pirates or anything better than this?

There’s actually a really neat trick that let’s you override any template from any bundle. All we need to do is create a template with the same name as this in just the right location.

This template lives in TwigBundle and in an Exception directory. Create an app/Resources/TwigBundle/views/Exception/error.html.twig file. Notice how similar the paths are - it’s the magic way to override any template from any bundle.


app/Resources/AnyBundle/views/SomeDir/myTemplate.html.twig will always override @AnyBundle/Resources/views/SomeDir/myTemplate.html.twig

Now just extend the base template and put something awesome inside. I’m going to abuse my login.css file to get this to look ok. I know, I really need to clean up my CSS:

{# app/Resources/TwigBundle/views/Exception/error.html.twig #}
{% extends '::base.html.twig' %}

{% block stylesheets %}
    {{ parent() }}
    <link rel="stylesheet" href="{{ asset('bundles/user/css/login.css') }}" />
{% endblock %}

{% block body %}
    <section class="login">
            <h1>Ah crap!</h1>

            <div>These are not the droids you're looking for...</div>
{% endblock %}

Refresh! Hey, don’t act so surprised to see the same ugly template. We’re in the prod environment, we need to clear our cache after every change:

php app/console cache:clear --env=prod

Refresh again. It’s beautiful. The pain with customizing error templates is that you need to be in the prod environment to see them. And that means you need to remember to clear cache after every change.

Customizing Error Pages by Type (Status Code)

But we have a problem: this template is used for all errors: 404 errors, 500 errors and even the dreaded 418 error!

I think we should at least have one template for 404 errors and another for everything else. Copy the existing template and paste it into a new file called error404.html.twig. That’s the trick, and this works for customizing the error page of any HTTP status code.

We should keep the generic error template, but let’s give it a different message:

{# app/Resources/TwigBundle/views/Exception/error.html.twig #}

{# ... #}
<h1>Ah crap!</h1>

<div>The servers are on fire! Grab a bucket! Send halp!</div>

To see the 404 template, clear your cache and refresh again on an imaginary URL. To see the other template, temporarily throw an exception in EventController::indexAction to cause a 500 error:

// src/Yoda/EventBundle/Controller/EventController.php
// ...

public function indexAction()
    throw new \Exception('Ahhhhahahhhah');
    // ...

Head to the homepage - but with the app.php still in the URL. You should see that the servers are in fact on fire, which I guess is cool. Remove this exception before moving on.

Going Deeper with Exception Handling

Behind the scenes, Symfony dispatches an event whenever an exception happens. We haven’t talked about events yet, but this basically means that if you want, you can be nofitied whenever an exception is thrown anywhere in your code. Why would you do this? You might want to do some extra logging or even completely replace which template is rendered when an error happens.

We won’t cover event listeners in this screencast, but there’s a cookbook called How to Create an Event Listener that covers it.

Normally, when there’s an exception, Symfony calls an internal controller that renders the error template. This class lives in TwigBundle and is called ExceptionController. Let’s open it up!

The class lives at: vendor/symfony/symfony/src/Symfony/Bundle/TwigBundle/Controller/ExceptionController.php

The guts of this class aren’t too important, but you can see it trying to figure out which template to render in findTemplate. You can even see it looking for the status-code version of the template, like error404.html.twig:

// vendor/symfony/symfony/src/Symfony/Bundle/TwigBundle/Controller/ExceptionController.php
// ...

$template = new TemplateReference('TwigBundle', 'Exception', $name.$code, $format, 'twig');
if ($this->templateExists($template)) {
    return $template;

I’m making you stare at this class because, if you want, you can actually override this entire controller. If you do that, then your controller function will be called whenever there’s an error and you can render whatever page you want. That process is a bit more involved, but use it if you need to go even further.