I stumbled upon this on upgrading the Flex recipe for symfony/framwork-bundle. The docs about Configuring Symfony makes it seem like XML and PHP config are supported by default:

Symfony lets you choose between YAML, XML and PHP and throughout the Symfony documentation, all configuration examples will be shown in these three formats.

XML and PHP config are not loaded inside the Kernel by default anymore since this commit in symfony/recipes. It is also not mentioned inside Configuring in the Kernel.

It might be useful to add some lines to the docs because this might be confusing to beginners.