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run code in 200+ php & hhvm versions
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<?php $key = null; var_dump(isset($key)); $key = 0; var_dump(isset($key)); $key = '0'; var_dump(isset($key));
Output for 4.3.0 - 5.6.30, hhvm-3.10.1 - 3.17.3, 7.0.0 - 7.2.0
bool(false) bool(true) bool(true)