3v4l.org

run code in 300+ PHP versions simultaneously
<?php $a = ["foo" => [1,2,3], "bar" => [5,2,3,4,3], "baz" => [5,2,1,6]]; echo json_encode(array_map(function ($k, $v) { array_splice($v, 0, 0, $k); return $v; }, array_keys($a), $a));

preferences:
64.55 ms | 402 KiB | 5 Q