Is reference counting necessary for a PHP implementation?

In: https://github.com/php/php-langspec/blob/master/spec/04-basic-concepts.md#reclamation-and-automatic-memory-management > Despite the use of the term refcount, conforming implementations are not required to use a reference counting-based implementation for automatic memory management. Is this statement correct? If I understand correctly many PHP projects depend on the deterministic firing of __destruct() function to cleanup SQL transactions or connections and so forth. HHVM elaborates on this: > Eliminating destructors. Deterministic object destruction is the reason why nonscalar PHP values require precise reference counting. This requirement has long been, and continues to be, a sizable performance bottleneck in our optimized JIT-compiled code. Using garbage collection instead could unlock measurable performance improvements, and the behavior of destructors could be closely imitated by a combination of try/finally and other new language constructs. (from http://hhvm.com/blog/2017/09/18/the-future-of-hhvm.html ) I'm curious about the answer here. Is ref counting necessary for all PHP implementations like HHVM claims? Thanks, Sidharth