Symfony 4.3

Symfony 4.3 added first-party HttpClient, Mailer, and String components. HttpClient provided a Symfony-native API for synchronous and asynchronous HTTP requests; Mailer provided a new email abstraction and transport system intended to replace Swiftmailer-based integrations over time; and String added Unicode-aware string manipulation utilities. The cycle also introduced deprecations and API adjustments that prepared applications and bundles for the following major-version transition.

Symfony 4.3 has reached end of life, so defects and security issues in this branch no longer receive vendor fixes. An application can also become increasingly difficult to operate as PHP versions, Composer dependencies, browsers, and infrastructure move forward: supported packages may drop compatibility, while packages retained for 4.3 can carry their own unpatched vulnerabilities. Remaining on 4.3 also postpones resolution of its deprecations, making a later major upgrade larger and riskier.

Inventory the application, its Symfony bundles, PHP requirement, and external integrations before changing constraints. Upgrade first to the latest compatible Symfony 4.4 release, use the deprecation reporting tools in development and test environments to remove 4.3 and 4.4 deprecations, and replace abandoned or incompatible bundles. Review email and HTTP integrations separately: plan a tested move from Swiftmailer-based code to Symfony Mailer where applicable, and validate any HTTP-client behavior such as timeouts, retries, authentication, and error handling. Once the application is clean on 4.4, follow the documented upgrade path to a currently maintained Symfony line, with automated regression, integration, and deployment tests covering the migration.