Symfony 3.4 provides a simpler way to inject all services tagged with a specific tag, so you don't have to create a compiler pass just to do that.
Symfony 3.4 adds new HTML5 form types ("tel" and "color"), new options for TimezoneType and support for callbacks in the delete_empty option.
In Symfony 3.4, the kernel of the application can implement CompilerPassInterface to define a compiler pass in it.
Symfony 3.4 adds a new form theme for Bootstrap 4, the most popular front-end component library.
Symfony 3.4 includes an Argon2i password hasher , a modern alternative to Bcrypt natively supported by PHP 7.2.
Symfony 3.4 provides new configuration options so you can generate the right URL for assets out of the HTTP context, for example in a command console.
Symfony 3.4 deprecates bundle inheritance, but Symfony still provides lots of alternatives to override the resources of third-party bundles in your application.
Symfony 3.4 adds a new CoverageListener to the PHPUnit Bridge component to create more precise code coverage reports.
Symfony 3.4 introduces a new (optional) PHP-based configuration format to define routes and services using a fluent interface.
Symfony 3.4 allows to define the data type of the value stored in environment variables and also defines some operators to transform their values before using them.