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

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

}

var_dump($C);

Warning: Cannot use a scalar value as an array on line 10
array(2) {
  ["EUR"]=>
  int(10)
  ["JPY"]=>
  array(2) {
    [0]=>
    array(0) {
    }
    [1]=>
    int(20)
  }
}