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. 3.0.0), 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. 2.8.*), you should be able to upgrade without a problem.

To help you with this, deprecation 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:

Of course ultimately, you want to stop using the deprecated functionality. Sometimes, this is easy: 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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$ phpunit
...

OK (10 tests, 20 assertions)

Remaining deprecation notices (6)

The "request" service is deprecated and will be removed in 3.0. Add a typehint 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="weak"/>
    </php>
</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:

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

    "require": {
        "symfony/symfony": "3.0.*",
    },
    "...": "..."
}

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

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$ composer update symfony/symfony

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$ composer update symfony/symfony --with-dependencies

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$ composer update

3) Update your Code to Work with the New Version

There is a good chance that you're done now! However, the next major version may also contain new BC breaks as a BC layer is not always a possibility. 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.