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<?php $arr = ["a", "b", "c", "d", "e", "f"]; foreach($arr as $k => $v) $arr[$k] = ["current" => $v, "next" => ""]; foreach($arr as $k => $v) { if(isset($arr[$k+1])) $arr[$k]["next"] = &$arr[$k+1]["current"]; else $arr[$k]["next"] = ""; } print_r($arr);
Output for 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.38, 7.0.0 - 7.0.33, 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 - 8.1.28, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
Array ( [0] => Array ( [current] => a [next] => b ) [1] => Array ( [current] => b [next] => c ) [2] => Array ( [current] => c [next] => d ) [3] => Array ( [current] => d [next] => e ) [4] => Array ( [current] => e [next] => f ) [5] => Array ( [current] => f [next] => ) )

preferences:
252.23 ms | 405 KiB | 340 Q