Recursive arrayAddition() by Miro Balearski

<?php

$a = [
    "ABC" => [
        "A" => 1000,
        "B" => 2000,
        "C" => ['D'=> 3000,
                'E'=> 4000,
                'G'=> ['Z'=> 787878]
                ]
            ]
    ];

$b = [
    "ABC" => [
        "A" => 100,
        "B" => 200,
        "C" => ['D'=> 300,
                'E'=> 400,
                'F'=> 500,
                'G'=> ['R'=> 777777,
                        'T'=> 888888,
                        'Y'=> 999999,

                       ]
               ]
    ]
];


function arrayAddition($old, $new){

    foreach($new as $kk=>$vv){
        if(is_array($vv)){
            if(array_key_exists($kk, $old)){
              $old[$kk]= arrayAddition($old[$kk], $new[$kk] );
            }else{
              $old[$kk]= $new[$kk];
            }

        }else{
            if(array_key_exists($kk, $old)){
                $old[$kk]= ($new[$kk] + $old[$kk]);
            }else{
                $old[$kk]= ($new[$kk]);
            }
        }
    }

    return $old;
}


$c = arrayAddition($a, $b);

print_r($a);
print_r($b);
print_r($c);