In a new tutorial Dalibor Karlović shows you how to test your Symfony application in production to get a more "real world" picture of how your application is performing for the rest of the world.
The problem here is that you almost cannot guarantee that you can replicate the production environment down to a single detail, it might have a slightly different underlying system, a slightly different network setup, even a different updates applied might mean it works for you, but not on production.
He starts the post by talking about the testing support already built into Symfony and the different parts tested by unit versus functional tests. He gets into some actual (functional) test examples, showing how to evaluate the response from an API request and where the major part of the overhead is - the database interaction. He takes the next step and looks at how to avoid "impure" functional testing and only then starts talking about switching between database types (SQLite vs MySQL) for better performance measurements. Finally, he gets to the topic of the article, running tests in production, and includes a "gotcha" to look out for (hint: don't hard-code IDs).