Symfony 5.4

Symfony 5.4 is the long-term-maintenance branch that completes the Symfony 5 generation while retaining its compatibility model. It adds further PHP 8-era integration, including native enum handling in relevant components such as forms and serialization, and includes features such as login-link authentication. More importantly for migration planning, it carries forward the deprecations introduced across the 5.x line and adds transition paths intended to make code compatible with Symfony 6. Applications can generally adopt 5.4 without the major API breaks associated with moving to the next major version, but deprecated APIs should be treated as migration work rather than permanent interfaces.

Remaining on 5.4 is not an immediate unsupported-dependency risk while this line remains supported, and it can be a sensible stable target for applications that still need its older PHP compatibility floor. The trade-off is that new Symfony capabilities, current component improvements, and compatibility with newer PHP baselines are concentrated in later major lines. A codebase that continues to use 5.4-era deprecated APIs, legacy security patterns, or outdated bundle dependencies will accumulate a larger upgrade gap. The eventual move will require addressing deprecations, validating third-party package compatibility, and potentially raising the PHP version, rather than being a routine package update.

Keep Symfony 5.4 patched within its supported release stream, then make the application clean under Symfony's deprecation reporting in development and automated tests. Inventory all Symfony packages, bundles, PHP extensions, and framework integrations; confirm that each has a maintained version compatible with the intended newer Symfony line. Replace deprecated Symfony APIs and configuration, especially in security and authentication code, before changing the framework constraint. Establish the target PHP version early, test the application and its deployment image against it, and perform the major-version upgrade in a separate branch with a full integration and production-like test pass.