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<?php $a = $r = $m = []; $a[1] = ['1','2','3']; $a[2] = ['4','5','6']; foreach ($a as $p) { $r = array_merge($r, $p); } var_dump($r); var_dump(array_map('array_merge', $a, [$r]));
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
array(6) { [0]=> string(1) "1" [1]=> string(1) "2" [2]=> string(1) "3" [3]=> string(1) "4" [4]=> string(1) "5" [5]=> string(1) "6" } Fatal error: Uncaught TypeError: array_merge(): Argument #2 must be of type array, null given in /in/1cG5s:9 Stack trace: #0 [internal function]: array_merge(Array, NULL) #1 /in/1cG5s(9): array_map('array_merge', Array, Array) #2 {main} thrown in /in/1cG5s on line 9
Process exited with code 255.
Output for 7.3.0 - 7.3.33, 7.4.0 - 7.4.33
array(6) { [0]=> string(1) "1" [1]=> string(1) "2" [2]=> string(1) "3" [3]=> string(1) "4" [4]=> string(1) "5" [5]=> string(1) "6" } Warning: array_merge(): Expected parameter 2 to be an array, null given in /in/1cG5s on line 9 array(2) { [0]=> array(9) { [0]=> string(1) "1" [1]=> string(1) "2" [2]=> string(1) "3" [3]=> string(1) "1" [4]=> string(1) "2" [5]=> string(1) "3" [6]=> string(1) "4" [7]=> string(1) "5" [8]=> string(1) "6" } [1]=> NULL }
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
array(6) { [0]=> string(1) "1" [1]=> string(1) "2" [2]=> string(1) "3" [3]=> string(1) "4" [4]=> string(1) "5" [5]=> string(1) "6" } Warning: array_merge(): Argument #2 is not an array in /in/1cG5s on line 9 array(2) { [0]=> array(9) { [0]=> string(1) "1" [1]=> string(1) "2" [2]=> string(1) "3" [3]=> string(1) "1" [4]=> string(1) "2" [5]=> string(1) "3" [6]=> string(1) "4" [7]=> string(1) "5" [8]=> string(1) "6" } [1]=> NULL }

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