> Before, I'd feel comfortable returning an array of structured data from a function. Now, I'm creating Value Objects to ensure my data is both valid and correctly used elsewhere.

Same. I've written some complicated ETL/data transform processes in PHP, and used arrays because the array_* functions are so useful for manipulation.

But keeping track of what keys were available was a nightmare - especially coming back to maintain the system.

Like you I've switched to Value Objects. In many ways it's a pain (the intermediate representation that I'd stick into a new array key and pass to the next transformation - it doesn't belong on any of the objects; and PHP needs the equivalent of the array_* functions for objects) but knowing I can't use the wrong key somewhere makes it worthwhile.