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<?php $a = ['a' => [1,2,3], 'b' => [4,5,6], 'c' => [7,8,9]]; var_dump( array_slice($a, 0, 2) + ['xxx' => [10,11,12]] + array_slice($a, 2, count($a) - 2) );
Output for 5.5.24 - 5.5.35, 5.6.8 - 5.6.28, 7.0.0 - 7.0.20, 7.1.0 - 7.1.7, 7.2.29 - 7.2.33, 7.3.16 - 7.3.33, 7.4.0 - 7.4.33, 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(4) { ["a"]=> array(3) { [0]=> int(1) [1]=> int(2) [2]=> int(3) } ["b"]=> array(3) { [0]=> int(4) [1]=> int(5) [2]=> int(6) } ["xxx"]=> array(3) { [0]=> int(10) [1]=> int(11) [2]=> int(12) } ["c"]=> array(3) { [0]=> int(7) [1]=> int(8) [2]=> int(9) } }

preferences:
155.96 ms | 405 KiB | 178 Q