<?php
use Iterator;
use Traversable;
use RecursiveArrayIterator;
use JsonSerializable;

use RecursiveIterator;

interface DataInterface extends JsonSerializable, RecursiveIterator, Iterator
{
    /**
     * @param mixed $data
     *
     * @return DataInterface
     */
    public static function init(Traversable $data): DataInterface;
}


class A extends RecursiveArrayIterator implements DataInterface
{
    public function __construct($data)
    {
        parent::__construct($data);
    }
    
    public static function init($data): DataInterface
    {
        return new static($data);
    }
    
    public function getCols(string $colname): array
    {
        return array_column($this->getArrayCopy(), $colname);
    }
    
    public function bugySetMethod($key, $value)
    {
        $data = &$this;

        while ($data->hasChildren()) {
            $data = $data->getChildren();
        }
        
        $data->offsetSet($key, $value);
    }
    
    public function jsonSerialize()
    {
        return $this;
    }
}

$example = A::init([
    'test' => [
        'a' => (object)[2 => '',3 => '',4 => ''],
    ]
]);

$example->bugySetMethod(5, 'in here');
var_dump(json_encode($example));

// result
// string(51) "{"test":{"a":{"2":"","3":"","4":"","5":"in here"}}}"

$example = A::init([
    'test' => [
        'b' => [2 => '',3 => '',4 => ''],
    ]
]);

$example->bugySetMethod(5, 'must be here');
var_dump(json_encode($example));

// result
// string(37) "{"test":{"b":{"2":"","3":"","4":""}}}"