<?php

$json = '{"result":{"stats":[{"balance":"1","rejected_speed":"0","accepted_speed":"0"},{"balance":"2","rejected_speed":"0","accepted_speed":"0"},
{"balance":"3.00038357","rejected_speed":"0","accepted_speed":"0"}]}}';
$data = json_decode($json);
echo array_sum(array_column($data->result->stats, 'balance'));