php 兩個數組，若鍵相同，則值合並

<?php

$arr1  = array('9' => '4.08', '10' => '0.10', '11' => '4.08', '12' => '0.01');
$arr2  = array('8' => '0.01', '10' => '0.01', '11' => '0.01');

$arr3 = $arr1 + $arr2;

$arr4 = array_intersect_key($arr2, $arr1);
foreach($arr4 as $k => $v) {
    $arr3[$k] += $v;
}

print_r($arr3);