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run code in 200+ php & hhvm versions
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<?php @$undef; //xdebug_debug_zval('undef'); $undef = "defined"; debug_zval_dump($undef);
based on LLV3V
Output for hhvm-3.10.1 - 3.17.3, 7.0.0 - 7.1.1, 7.1.7 - 7.2.0
string(7) "defined" refcount(1)
Output for 7.1.2 - 7.1.6
string(7) "defined" refcount(3)
Output for 4.3.0 - 5.6.30
string(7) "defined" refcount(2)