Use \PHPUnit_Framework_TestCase or ..\TestCase

Stupid question... In the Symfony docs, unit test classes extend `\PHPUnit_Framework_TestCase` but in PHPUnit docs, test classes extend `PHPUnit\Framework\TestCase`. Is there a difference, are they the same, or am I missing a subtle PHP feature where `_` is equivalent to `\`? Short of actually testing it out myself, can I use either, or just one in Symfony? Update: It's been helpfully explained to me that these two classes are just a difference between PHPUnit versions. After my own research and for the record, Symfony 3.1 still extends `\PHPUnit_Framework_TestCase` in its own `KernelTestCase` class that powers Symfony's functional testing. To avoid breaking code, I'm going to play safe & extend `WebTestCase` that inherits from `\PHPUnit_Framework_TestCase` when unit testing with Symfony specifically. Such unit tests still work with PHPUnit 5.6 anyway