Description:
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I noticed that if you do json_encode() on a JsonSerializable object, it always populates the properties hashtable, because php_json_encode_serializable_object() needs a hashtable to have somewhere to set the recursion flag.

You might think it could just set the recursion flag on the object, but a few test cases (and probably users) do var_dump($this) in jsonSerialize(), which breaks if you protect the object prior to calling jsonSerialize().

It's possible to solve this by adding a second recursion flag to gc_flags, but that's a bit of a joke solution since I wouldn't expect a PR which consumes a bit in gc_flags to be accepted. But here it is if you're curious: 
https://github.com/tstarling/php-src/commit/1e975dfa731ceba4533aea0efdbd803d226963c9