<?php
$A = array("EUR"=>10,"USD"=>20);
$B = array("EUR"=>10,"JPY"=>20);

$C = $A;
foreach ($B as $key => $value) {
    if (isset($C[$key])) {
        $C[$key] = $C[$key] + $value;
    } else {
        $C[$key] = $value;
    }
}

var_dump($C);

array(3) {
  ["EUR"]=>
  int(20)
  ["USD"]=>
  int(20)
  ["JPY"]=>
  int(20)
}