- array_map: documentation ( source)
- array_keys: documentation ( source)
- json_encode: documentation ( source)
<?php
$a = ["foo" => [1,2,3], "bar" => [5,2,3,4,3], "baz" => [5,2,1,6]];
echo json_encode(array_map(function ($k, $v) { return [$k, ...$v]; }, array_keys($a), $a));