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<?php $a = ['a' => ['1','2']]; $b = ['a' => ['3','4']]; var_dump(array_merge_recursive($a, $b)['a']); var_dump($a['a'] + $b['a']);
Output for 5.5.24 - 5.5.35, 5.6.8 - 5.6.28, 7.0.0 - 7.0.20, 7.1.0 - 7.1.25, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
array(4) { [0]=> string(1) "1" [1]=> string(1) "2" [2]=> string(1) "3" [3]=> string(1) "4" } array(2) { [0]=> string(1) "1" [1]=> string(1) "2" }

preferences:
174.52 ms | 404 KiB | 192 Q