Phil Pichet worked on enhancing HTTP caching mechanisms within the symfony/http-kernel and symfony/symfony repositories, focusing on correct handling of multiple Vary headers. Using PHP and leveraging backend development and HTTP protocol expertise, Phil implemented array-based processing for Vary headers, ensuring cache lookups accurately reflect varied request headers. The approach included joining multiple Vary headers into a single comma-separated string, aligning cache store behavior with HTTP standards. Comprehensive tests were added to cover Vary header variations, reducing cache misses and improving response correctness. This work deepened the reliability of Symfony’s caching layer and reduced backend load for varied client requests.