1) Make your Code Deprecation Free

During the lifecycle of a major release, new features are added and method signatures and public API usages are changed. However, minor versions should not contain any backwards incompatible changes. To accomplish this, the "old" (e.g. functions, classes, etc) code still works, but is marked as deprecated, indicating that it will be removed/changed in the future and that you should stop using it.

When the major version is released (e.g. 6.0.0), all deprecated features and functionality are removed. So, as long as you've updated your code to stop using these deprecated features in the last version before the major (e.g. 5.4.*), you should be able to upgrade without a problem. That means that you should first upgrade to the last minor version (e.g. 5.4) so that you can see all the deprecations.

To help you find deprecations, notices are triggered whenever you end up using a deprecated feature. When visiting your application in the dev environment in your browser, these notices are shown in the web dev toolbar:

Ultimately, you should aim to stop using the deprecated functionality. Sometimes the warning might tell you exactly what to change.

But other times, the warning might be unclear: a setting somewhere might cause a class deeper to trigger the warning. In this case, Symfony does its best to give a clear message, but you may need to research that warning further.

And sometimes, the warning may come from a third-party library or bundle that you're using. If that's true, there's a good chance that those deprecations have already been updated. In that case, upgrade the library to fix them.

Once all the deprecation warnings are gone, you can upgrade with a lot more confidence.

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$ composer require --dev symfony/phpunit-bridge

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# this command is available after running "composer require --dev symfony/phpunit-bridge"
$ ./bin/phpunit
...

OK (10 tests, 20 assertions)

Remaining deprecation notices (6)

The "request" service is deprecated and will be removed in 3.0. Add a type-hint for
Symfony\Component\HttpFoundation\Request to your controller parameters to retrieve the
request instead: 6x
    3x in PageAdminTest::testPageShow from Symfony\Cmf\SimpleCmsBundle\Tests\WebTest\Admin
    2x in PageAdminTest::testPageList from Symfony\Cmf\SimpleCmsBundle\Tests\WebTest\Admin
    1x in PageAdminTest::testPageEdit from Symfony\Cmf\SimpleCmsBundle\Tests\WebTest\Admin

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<!-- phpunit.xml.dist -->
<phpunit>
    <!-- ... -->

    <php>
        <env name="SYMFONY_DEPRECATIONS_HELPER" value="max[total]=999999"/>
    </php>
</phpunit>

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$ SYMFONY_DEPRECATIONS_HELPER=max[total]=999999 php ./bin/phpunit

2) Update to the New Major Version via Composer

Once your code is deprecation free, you can update the Symfony library via Composer by modifying your composer.json file and changing all the libraries starting with symfony/ to the new major version:

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{
      "...": "...",

      "require": {
-         "symfony/cache": "5.4.*",
+         "symfony/cache": "6.0.*",
-         "symfony/config": "5.4.*",
+         "symfony/config": "6.0.*",
-         "symfony/console": "5.4.*",
+         "symfony/console": "6.0.*",
          "...": "...",

          "...": "A few libraries starting with
                  symfony/ follow their own versioning scheme. You
                  do not need to update these versions: you can
                  upgrade them independently whenever you want",
          "symfony/monolog-bundle": "^3.5",
      },
      "...": "...",
  }

At the bottom of your composer.json file, in the extra block you can find a data setting for the Symfony version. Make sure to also upgrade this one. For instance, update it to 6.0.* to upgrade to Symfony 6.0:

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"extra": {
      "symfony": {
          "allow-contrib": false,
-       "require": "5.4.*"
+       "require": "6.0.*"
      }
  }

Next, use Composer to download new versions of the libraries:

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$ composer update "symfony/*"

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$ composer update "symfony/*" --with-all-dependencies

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$ composer update

3) Updating Recipes

Over time - and especially when you upgrade to a new version of a library - an updated version of the recipe may be available. These updates are usually minor - e.g. new comments in a configuration file - but it's a good idea to keep your files in sync with the recipes.

Symfony Flex provides several commands to help upgrade your recipes. Be sure to commit any unrelated changes you're working on before starting:

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# choose an outdated recipe to update
$ composer recipes:update

# update a specific recipe
$ composer recipes:update symfony/framework-bundle

# see a list of all installed recipes and which have updates available
$ composer recipes

# see detailed information about a specific recipes
$ composer recipes symfony/framework-bundle

The recipes:update command is smart: it looks at the difference between the recipe when you installed it and the latest version. It then creates a patch and applies it to your app. If there are any conflicts, you can resolve them like a normal git conflict and commit like normal.

4) Update your Code to Work with the New Version

In some rare situations, the next major version may contain backwards-compatibility breaks. Make sure you read the UPGRADE-X.0.md (where X is the new major version) included in the Symfony repository for any BC break that you need to be aware of.

Upgrading to Symfony 6: Add Native Return Types

Symfony 6 will come with native PHP return types to (almost all) methods.

In PHP, if the parent has a return type declaration, any class implementing or overriding the method must have the return type as well. However, you can add a return type before the parent adds one. This means that it is important to add the native PHP return types to your classes before upgrading to Symfony 6.0. Otherwise, you will get incompatible declaration errors.

When debug mode is enabled (typically in the dev and test environment), Symfony will trigger deprecations for every incompatible method declarations. For instance, the UserInterface::getRoles() method will have an array return type in Symfony 6. In Symfony 5.4, you will get a deprecation notice about this and you must add the return type declaration to your getRoles() method.

To help with this, Symfony provides a script that can add these return types automatically for you. Make sure you installed the symfony/error-handler component. When installed, generate a complete class map using Composer and run the script to iterate over the class map and fix any incompatible method:

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# Make sure "exclude-from-classmap" is not filled in your "composer.json". Then dump the autoloader:

# "-o" is important! This forces Composer to find all classes
$ composer dump-autoload -o

# patch all incompatible method declarations
$ ./vendor/bin/patch-type-declarations

The behavior of this script can be modified using the SYMFONY_PATCH_TYPE_DECLARATIONS env var. The value of this env var is url-encoded (e.g. param1=value1&param2=value2), the following parameters are available:

force

Enables fixing return types, the value must be one of:

• 2 to add all possible return types (default, recommended for applications);
• 1 to add return types only to tests, final, internal or private methods;
• phpdoc to only add @return docblock annotations to the incompatible methods, or #[\ReturnTypeWillChange] if it's triggered by the PHP engine.

php

The target version of PHP - e.g. 7.1 doesn't generate "object" types (which were introduced in 7.2). This defaults to the PHP version used when running the script.

deprecations

Set to 0 to disable deprecations. Otherwise, a deprecation notice when a child class misses a return type while the parent declares an @return annotation (defaults to 1).

If there are specific files that should be ignored, you can set the SYMFONY_PATCH_TYPE_EXCLUDE env var to a regex. This regex will be matched to the full path to the class and each matching path will be ignored (e.g. SYMFONY_PATCH_TYPE_EXCLUDE="/tests\/Fixtures\//"). Classes in the vendor/ directory are always ignored.