DOC Add provisional 4.7.0 changelog [ci skip]

docs/en/04_Changelogs/4.7.0.md

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+# 4.7.0 (Unreleased)
+
+## Overview
+
+- [Experimental support for PHP 8](#experimental-support-for-php-8)
+- [Support for Symfony 4 Components](#support-for-symfony-4-components)
+- [Default MySQL collation updated](#default-mysql-collation-updated)
+
+## New features
+
+### Experimental support for PHP 8
+
+You can now run Silverstripe CMS on PHP 8, which is due for release in November 2020. PHP 8 includes
+many exciting new features like named parameters, attributes, and union types. There are a few
+important caveats to keep in mind if you plan to adopt PHP 8 at release.
+
+Upgrading to PHP 8 on projects using `SapphireTest` for unit testing requires adoption of the
+[`sminnee/phpunit`](https://packagist.org/packages/sminnee/phpunit) fork, and Prophecy is not
+currently supported. Prophecy tests can be fairly trivially ported to PHPUnit Mock Objects.  We hope
+to give you better support for choosing your own testing tools, including newer versions of PHPUnit,
+in a future release.
+
+Support for PHP 8 does not yet extend to non-core modules, and PHP 8 itself is not yet stable, so
+compatibility issues could arise in future PHP builds. This extends to dependencies of core, many of
+which have not yet declared support for PHP 8, so you will need to run Composer with the
+`--ignore-platform-reqs` flag for the time being.
+
+Finally, support for the new JIT functionality has not yet been tested.
+
+We encourage early adopters to report any issues running CMS Recipe 4.7.0 against PHP 8 on the
+relevant GitHub repository.
+
+### Support for Symfony 4 Components
+
+Symfony 3 will become unsupported early next year, so Silverstripe CMS is now forwards-compatible
+with Symfony 4 components. This is a largely transparent upgrade, but you may encounter errors
+related to strings starting with `%` in YML files, which can be easily rectified by wrapping all
+affected strings in quotes.
+
+If you'd rather retain the previous YAML parser for the time being, you can run
+`composer require symfony/yaml:^3` on your project to prevent the update to version 4.
+
+### Default MySQL collation updated
+
+New projects based on `silverstripe/installer` will default to the `utf8mb4_unicode_ci` collation.
+This change will not affect existing projects, but developers are encouraged to adopt this collation
+as it provides better support for multi-byte characters such as emojis.
+
+Depending on the version of MySQL you are running, you may encounter issues with `Varchar` fields
+exceeding the maximum indexable size:
+
+- MySQL 5.5 and lower cannot support indexes larger than 768 bytes (192 characters)
+- MySQL 5.6 supports larger indexes (3072 bytes) if the `innodb_large_prefix` setting is enabled (not by default)
+- MySQL 5.7 and newer have `innodb_large_prefix` enabled by default
+- MariaDB ~10.1 matches MySQL 5.6's behaviour, >10.2 matches 5.7's.
+
+You can rectify this by upgrading MySQL, enabling the `innodb_large_prefix` setting if present, or
+reducing the size of affected fields. If none of these solutions are currently suitable, you can
+remove the new collation configuration to default back to the previous default collation.