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<?php function couple($items) { $a = array(); for ($i = 0, $n = count($items); $i < $n; $i++) for ($j = $i + 1; $j < $n; $j++) $a[] = array($items[$j], $items[$i]); return $a; } var_dump(couple(array(1,2,3))) ?>
Output for 4.3.0 - 4.3.11, 4.4.0 - 4.4.9, 5.0.0 - 5.0.5, 5.1.0 - 5.1.6, 5.2.0 - 5.2.17, 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.40, 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 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 - 8.1.28, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
array(3) { [0]=> array(2) { [0]=> int(2) [1]=> int(1) } [1]=> array(2) { [0]=> int(3) [1]=> int(1) } [2]=> array(2) { [0]=> int(3) [1]=> int(2) } }

preferences:
265.86 ms | 406 KiB | 460 Q