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run code in 150+ php & hhvm versions
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<?php $o = new stdClass; $o->self = $o; echo serialize($o);
Output for 5.0.0 - 7.1.0
O:8:"stdClass":1:{s:4:"self";r:1;}
Output for 4.3.0 - 4.4.9
O:8:"stdClass":1:{s:4:"self";O:8:"stdClass":0:{}}