Bidirectional mapping model in PHP

bidir_map.php

<?php
interface iBidirectionTrans {
    public function forward_map($left_val);
    public function reverse_map($right_val, &$left_obj);
}

class KeyMapper implements iBidirectionTrans {
    private $pos;
    public function __construct($pos) {
        $this->pos = $pos;
    }
    public function forward_map($left_val) {
        return $left_val[$this->pos];
    }
    public function reverse_map($right_val, &$left_obj) {
        $left_obj[$this->pos] = $right_val;
    }
}

class ArrayMapper implements iBidirectionTrans {
    private $arr;
    public function __construct($arr) {
        assert(is_array($arr));
        $this->arr = $arr;
    }
    public function forward_map($left_val) {
        return array_map(function($mapper) use ($left_val) {
            if (is_subclass_of($mapper, "iBidirectionTrans"))
                return $mapper->forward_map($left_val);
            else
                return $mapper;
        }, $this->arr);
    }
    public function reverse_map($right_val, &$left_obj) {
        foreach ($this->arr as $key => $value) {
            if (!isset($right_val[$key]))
                error_log("Key $key not found in right");
            if (is_subclass_of($value, "iBidirectionTrans"))
                $value->reverse_map($right_val[$key], $left_obj);
            else
                if ($right_val[$key] != $value) 
                    error_log("Value of key $key is not satisfied($value expected, ${right_val[$key]} got)");
        }
    }
}

$__0 = new KeyMapper(0);
$__1 = new KeyMapper(1);
$__2 = new KeyMapper(2);


$model = new ArrayMapper(
    [
        'abc' => $__0,
        'ccc' => new ArrayMapper(
            [
                'kcd' => 'fdsa',
                'xxx' => $__1
            ]
        )
    ]
);

$right = $model->forward_map([123, 345]);
print_r($right);
// Array ( [abc] => 123 [ccc] => Array ( [kcd] => fdsa [xxx] => 345 ) ) 

$right['abc'] = 789;
$right['ccc']['xxx'] = 0;
$left = [];
$model->reverse_map($right, $left);

print_r($left);
// Array ( [0] => 789 [1] => 0 )