Doctrine codebases calling `interface_exists` at the end of single-class/interface files?

I spotted this pattern in Doctrine codebases. E.g.: https://github.com/doctrine/data-fixtures/blob/b528aa964b5965d958f0f533df2ed611520dad5c/lib/Doctrine/Common/DataFixtures/FixtureInterface.php

Why is that interface_exists call done at the end of the file? What does it achieve? Is it some optimization or just a safeguard for certain situations?

There are other similar spots in the Doctrine codebases, not sure if it is in use in other projects.