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<?php $default = [ 'value' => '', ]; $input = [ 'value' => null, ]; var_dump(array_merge($default, $input));
Output for 7.4.0 - 7.4.33, 8.0.1 - 8.0.30, 8.1.0 - 8.1.29, 8.2.0 - 8.2.20, 8.3.0 - 8.3.8
array(1) { ["value"]=> NULL }

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125.87 ms | 404 KiB | 126 Q