Rob Allen has a post to his site showing how you can filter the body content of your response in a PSR-7 compatible system via some simple middleware.

Sometimes, there's a requirement to alter the data in the Response's body after it has been created by your controller action. For example, we may want to ensure that our brand name is consistently capitalised. One way to do this is to create middleware that [uses str_ireplace to replace a string]. That's great, but what happens if the new string is shorter than the old one? For instance, suppose I want to replace all uses of "nineteen feet" with "19FT".

In his example, replacing that part of the content would result in an odd string because of how they'd be replaced in the stream. He shows how to use the in-memory handling of PHP's fopen to create a new content instance to push back into the response and back out to the waiting application.