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<?php $a = array(1, 2, 3, 4, 5); $b = array_keys(function($i) { return $a[$i]*2; }); var_dump($b);
Output for 8.0.0 - 8.0.30, 8.1.0 - 8.1.28, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
Fatal error: Uncaught TypeError: array_keys(): Argument #1 ($array) must be of type array, Closure given in /in/Zhtn6:5 Stack trace: #0 /in/Zhtn6(5): array_keys(Object(Closure)) #1 {main} thrown in /in/Zhtn6 on line 5
Process exited with code 255.
Output for 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.40, 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33
Warning: array_keys() expects parameter 1 to be array, object given in /in/Zhtn6 on line 5 NULL
Output for 4.4.2 - 4.4.9, 5.1.0 - 5.1.6, 5.2.0 - 5.2.17
Parse error: syntax error, unexpected T_FUNCTION, expecting ')' in /in/Zhtn6 on line 5
Process exited with code 255.
Output for 4.3.0 - 4.3.1, 4.3.5 - 4.3.11, 4.4.0 - 4.4.1, 5.0.0 - 5.0.5
Parse error: parse error, unexpected T_FUNCTION, expecting ')' in /in/Zhtn6 on line 5
Process exited with code 255.
Output for 4.3.2 - 4.3.4
Parse error: parse error, expecting `')'' in /in/Zhtn6 on line 5
Process exited with code 255.

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