两个数组合并，保持两个数组的键值不变

$arr1 = ['a' => '111', 'b' => '222', 'c' => '333'];
$arr2 = ['d' => '444', 'e' => '555', 'f' => '666'];

$arr = $arr1 + $arr2;

输出结果：

array(6) {
  ["a"] => string(3) "111"
  ["b"] => string(3) "222"
  ["c"] => string(3) "333"
  ["d"] => string(3) "444"
  ["e"] => string(3) "555"
  ["f"] => string(3) "666"
}