PHP JSON convert null to string

safejson.php

<?php
$data = [
    "a" => 1,
    "b" => null,
    "c" => [
        "d" => 2,
        "e" => null,
        "f" => [
            "g" => 3,
            "h" => null
        ]
    ],
    null
];

function safeJson($arr){
    array_walk_recursive($arr, function (&$item, $key) {
        $item = is_null($item) ? '' : $item;
    });
    return $arr;
}

echo json_encode(safeJson($data));