php 数组合并问题 求解

PHP code:

$array=array("a","b");
$arr=array("c","d");
$c=$array+$arr;
print_r($c);



打印出来是Array ( [0] => a [1] => b ) //问题1

PHP code:

$array=array("a"=>"a","b"=>"b");
$arrays=array("c"=>"c","d"=>"d");
$c=$array+$arrays;
print_r($c);



打印出来是Array ( [a] => a [b] => b [c] => c [d] => d ) //问题2
为什么会有这样的差异 直接用+合并数组又要注意点什么问题 求解 谢谢

---

有问题先看看手册。
手册上的：
PHP code:

<?php
$array1 = array("color" => "red", 2, 4);
$array2 = array("a", "b", "color" => "green", "shape" => "trapezoid", 4);
$result = array_merge($array1, $array2);
print_r($result);
?>


上例将输出：

PHP code:
Array
(
[color] => green
[0] =>