One of the most time-consuming parts of writing tests is writing the code to set the world up in a known state and then return it to its original state when the test is complete. This known state is called the fixture of the test.
Writing your fixtures with PHP can quickly be a problem: you will have to write a huge amount of code, even many files. To reduce this pain you should use Alice. Alice allows you to create a ton of fixtures/fake data for use while developing or testing your project. To use it in a Symfony project go here.