Description "IntlExtension" memoises every "\IntlDateFormatter" and "\NumberFormatter" it creates in instance-level arrays keyed on a hash that includes "locale", "pattern", "attrs" and other values that are ordinary named arguments of the "format_datetime" / "format_date" / "format_time" / "format_number" / "format_currency" filters. There is no size limit and no eviction. A template that iterates over many distinct "pattern" (or "locale", or "grouping_used", ...) values therefore allocates one ICU formatter object per distinct value and pins it for the entire lifetime of the "Twig\Environment". Because ICU allocates its backing buffers outside the Zend memory manager, this growth is not bounded by "memory_limit". On long-running runtimes (RoadRunner, Swoole, FrankenPHP worker mode, ReactPHP) where the "Environment" outlives a single request, the cache also accumulates across requests. Resolution The formatter caches are now bounded in size (100 entries each) and evict on a FIFO basis. Credits Twig would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue and providing the fix.