The change log describes what is "Added", "Removed", "Changed" or "Fixed" between each release.
PHPUnit\Framework\TestCaseAbstractCachePool has 4 new abstract methods: getList, removeList, appendListItem and removeListItem.AbstractCachePool::invalidateTags and AbstractCachePool::invalidateTagsPhpCachePool and PhpCacheItemTagSupportWithArray.AbstractCachePool::storeItemInCache must be a PhpCacheItem.AbstractCachePool::fetchObjectFromCache must be a an array with 4 values. Added expiration timestamp.HasExpirationDateInterface is replaced by HasExpirationTimestampInterface\DateTime internally anymore. We work with timestamps.LoggerAwareInterfaceAbstractCachePool does not longer implement TaggablePoolInterface. However, the CacheItem does still implement TaggableItemInterface.CacheItem::getKeyFromTaggedKey has been removedCacheItem's second parameter is a callable that must return an array with 3 elements; [hasValue, value, tags].