According to Symfony Release Process, every two years Symfony releases the
last version of a branch (X.4) and the first version of the next branch
(Y.0) at the same time. That will happen at the end of November 2021,
when both Symfony 5.4 and Symfony 6.0 will be released.

The main difference between them is that Symfony 5.4 will still contain all
deprecated features and you can use it in applications using those deprecated
features. Symfony 6.0 removes all deprecated features. You’ll need to upgrade
to 5.4 first, remove all deprecations in your code and then upgrade to 6.0.

Introducing PHP Types Everywhere

This time, both Symfony 5.4 and 6.0 will include another change that might have
a big impact in your applications: they add PHP types in all properties,
arguments and return values of methods whenever possible.

This was a titanic multi-year effort led by Nicolas Grekas and
Alexander M. Turek that could be completed in time for Symfony 5.4 and 6.0.

The following is a real example of the impact of this change in your
applications. If you have a user class implementing Symfony’s
UserInterface, your code will probably include this:

This is no longer correct in Symfony 5.4/6.0. The reason is that Symfony has
added the array return type to the getRoles() method. Because of how
types work in PHP, this means that it’s mandatory for you to add the return type
in your code:

Adding PHP Types in your Own Code

Adding all the required PHP types in your applications can be a time-consuming
and cumbersome task. In order to make it easier, Symfony will provide several
tools:

When upgrading to Symfony 5.4, you’ll see deprecation messages whenever a
missing type in your code would cause a PHP error when upgrading;
The ErrorHandler component includes a little command utility called
patch-type-declarations which automatically adds the required return types
in your code;
For third-party open source packages/bundles, the upgrade process is similar
but you need to follow some extra steps to avoid backward compatibility breaks.

The whole upgrade process for Symfony apps and open source bundles is
explained in detail in the following blog post published by Wouter de Jong,
a Symfony Core Team member:

Symfony 6: PHP 8 Native Types & Why we Need YOU

If you are an open source package/bundle developer we need your help.
Please, test your bundles with Symfony 5.4 and 6.0 as soon as possible and
report any problems in this meta issue in the Symfony repository. Depending
on the received feedback, we might postpone some PHP type changes until Symfony
7.0. Thanks for your help!

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