Symfony 6.4

Symfony 6.4 is the long-term-support minor line after 6.3. It retains Symfony’s minor-version backward-compatibility approach while adding APIs and expanding deprecations for code that must change before a later major upgrade. The dependency-injection component added the AutowireIterator and AutowireLocator attributes, allowing tagged service iterables and service locators to be injected declaratively. The PHP baseline remains PHP 8.1, so moving from 6.3 does not inherently require a PHP-version increase.

This line remains actively supported, so staying on it does not itself remove vendor maintenance coverage. The trade-off is that new framework capabilities land in later lines, while application and bundle code may increasingly target newer Symfony or PHP requirements. Deprecation warnings ignored on 6.4 become concrete migration work when moving to a later major version. A locked Composer dependency set also needs regular maintenance updates; otherwise applicable Symfony and third-party fixes are not actually deployed.

Update the Symfony packages together to the latest available 6.4 patch level, including compatible dependency updates, and verify that the production PHP runtime and Composer platform configuration meet the PHP 8.1 minimum. Run the full test suite with deprecations reported, using the Symfony PHPUnit Bridge where applicable, then fix first-party deprecations and update bundles that emit or depend on deprecated APIs. Inventory Composer dependencies for their compatibility with the intended later Symfony major, resolve blockers while still on 6.4, and perform that major upgrade in a separate tested change set after the deprecation report is clean or explicitly understood.