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run code in 150+ php & hhvm versions
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<?php $arr = array ( 0 => 'nested array', 1 => 'nested array', 2 => 'nested array', 3 => 'nested array', 4 => 'nested array', 5 => 'nested array', ); var_dump(key(array_diff(array_keys(array_values($arr)), array_keys($arr))));
Output for 4.3.0 - 5.0.4, 5.1.0 - 7.1.0
NULL
Output for 5.0.5
Fatal error: Only variables can be passed by reference in /in/Tm7Ua on line 12
Process exited with code 255.