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<?php var_export(array_merge(['product' => 'p'], null));
Output for 7.3.0rc3
Warning: array_merge(): Expected parameter 2 to be an array, null given in /in/IfPuG on line 3 NULL
Output for 5.6.38
Warning: array_merge(): Argument #2 is not an array in /in/IfPuG on line 3 NULL