Allowing to Install Symfony 3 Components

Most third-party bundles define their Symfony dependencies using the ~2.N or ^2.N constraints in the composer.json file. For example:

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{
    "require": {
        "symfony/framework-bundle": "~2.7",
        "symfony/finder": "~2.7",
        "symfony/validator": "~2.7"
    }
}

These constraints prevent the bundle from using Symfony 3 components, which means the bundle cannot be installed in a Symfony 3 based application. Thanks to the flexibility of Composer dependencies constraints, you can specify more than one major version by replacing ~2.N by ~2.N|~3.0 (or ^2.N by ^2.N|~3.0).

The above example can be updated to work with Symfony 3 as follows:

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{
    "require": {
        "symfony/framework-bundle": "~2.7|~3.0",
        "symfony/finder": "~2.7|~3.0",
        "symfony/validator": "~2.7|~3.0"
    }
}

Look for Deprecations and Fix Them

Besides allowing users to use your bundle with Symfony 3, your bundle must stop using any feature deprecated by the 2.8 version because they are removed in 3.0 (you'll get exceptions or PHP errors). The easiest way to detect deprecations is to install the symfony/phpunit-bridge package and then run the test suite.

First, install the component as a dev dependency of your bundle:

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$ composer require --dev symfony/phpunit-bridge

Then, run your test suite and look for the deprecation list displayed after the PHPUnit test report:

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

# ... PHPUnit output

Remaining deprecation notices (3)

The "pattern" option in file ... is deprecated since version 2.2 and will be
removed in 3.0. Use the "path" option in the route definition instead ...

Twig Function "form_enctype" is deprecated. Use "form_start" instead in ...

The Symfony\Component\Security\Core\SecurityContext class is deprecated since
version 2.6 and will be removed in 3.0. Use ...

Fix the reported deprecations, run the test suite again and repeat the process until no deprecation usage is reported.

Testing your Bundle in Symfony 3

Now that your bundle has removed all deprecations, it's time to test it for real in a Symfony 3 application. Assuming that you already have a Symfony 3 application, you can test the updated bundle locally without having to install it through Composer.

If your operating system supports symbolic links, instead point the appropriate vendor directory to your local bundle root directory:

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$ ln -s /path/to/your/local/bundle/ vendor/you-vendor-name/your-bundle-name

If your operating system doesn't support symbolic links, you'll need to copy your local bundle directory into the appropriate directory inside vendor/.

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language: php
php:
    - 5.3
    - 5.6
    - 7.0

matrix:
    include:
        - php: 5.3.3
          env: COMPOSER_FLAGS='--prefer-lowest --prefer-stable' SYMFONY_DEPRECATIONS_HELPER=max[total]=999999
        - php: 5.6
          env: SYMFONY_VERSION='2.7.*'
        - php: 5.6
          env: SYMFONY_VERSION='2.8.*'
        - php: 5.6
          env: SYMFONY_VERSION='3.0.*'
        - php: 5.6
          env: SYMFONY_VERSION='3.1.*'
        - php: 5.6
          env: DEPENDENCIES='dev' SYMFONY_VERSION='3.2.*@dev'

before_install:
    - composer self-update
    - if [ "$DEPENDENCIES" == "dev" ]; then perl -pi -e 's/^}$/,"minimum-stability":"dev"}/' composer.json; fi;
    - if [ "$SYMFONY_VERSION" != "" ]; then composer --no-update require symfony/symfony:${SYMFONY_VERSION}; fi;

install: composer update $COMPOSER_FLAGS

script: phpunit

Updating your Code to Support Symfony 2.x and 3.x at the Same Time

The real challenge of adding Symfony 3 support for your bundles is when you want to support both Symfony 2.x and 3.x simultaneously using the same code. There are some edge cases where you'll need to deal with the API differences.

Before diving into the specifics of the most common edge cases, the general recommendation is to not rely on the Symfony Kernel version to decide which code to use:

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if (Kernel::VERSION_ID < 20800) {
    // code for Symfony 2.x
} else {
    // code for Symfony 3.x
}

Instead of checking the Symfony Kernel version, check the version of the specific component. For example, the OptionsResolver API changed in its 2.6 version by adding a setDefined() method. The recommended check in this case would be:

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use Symfony\Component\OptionsResolver\OptionsResolver;

if (!method_exists(OptionsResolver::class, 'setDefined')) {
    // code for the old OptionsResolver API
} else {
    // code for the new OptionsResolver API
}