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run code in 150+ php & hhvm versions
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<?php $arr = array( 'foo' => 'bar', 'bar' => $arr['foo'] ); var_dump($arr);
based on bgNju
Output for 5.0.4 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
Notice: Undefined variable: arr in /in/OouLA on line 4 array(2) { ["foo"]=> string(3) "bar" ["bar"]=> NULL }
Output for 4.3.0 - 5.0.3
Notice: Undefined variable: arr in /in/OouLA on line 4 array(2) { ["foo"]=> string(3) "bar" ["bar"]=> NULL }