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<?php $arr=Array ( "1" => Array ( "1" => "c1", "2" => "c1", "3" => "c1", "4" => "c1", "5" => "c1", "6" => "c1", "7" => "c1", "8" => "c1", "9" => "c1", "10" => "c1", "11" => "c1", "12" => "c1" ), "10" => Array ( "1" => "c2", "2" => "c2", "3" => "c2", "4" => "c2", "5" => "c2", "6" => "c2", "7" => "c2", "8" => "c2", "9" => "c2", "10" => "c2", "11" => "c2", "12" => "c2" ) ); echo json_encode( array_map( null, array_keys(current($arr)), ...$arr ), JSON_PRETTY_PRINT );
Output for 7.4.0 - 7.4.33, 8.0.1 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
[ [ 1, "c1", "c2" ], [ 2, "c1", "c2" ], [ 3, "c1", "c2" ], [ 4, "c1", "c2" ], [ 5, "c1", "c2" ], [ 6, "c1", "c2" ], [ 7, "c1", "c2" ], [ 8, "c1", "c2" ], [ 9, "c1", "c2" ], [ 10, "c1", "c2" ], [ 11, "c1", "c2" ], [ 12, "c1", "c2" ] ]

preferences:
126.07 ms | 403 KiB | 117 Q