collectdfs.php

<?php


$tree = array(
    "name" => "A", "child" => array(
        "name" => "B", "child" => array(
            "name" => "C", "child" => Null
        ),
    ),
);

function collect($node, $result=[]) {
    array_push($result, $node["name"]);
    if (!$node["child"]) {
        return $result;
    }
    $result = collect($node["child"], $result);
    return $result;
}

$result = collect($tree);

print_r($result);