What does array $arr += array $arr1 do?

[Krimson]

I've seen the following often lately and I'm wondering what it does? I can't seem to find it in the PHP manual.\[code\] $arr1 = array('key' => 'value1'); $arr2 = array('key' => 'value2'); $arr1 += $arr2;\[/code\]Is it similar to an array_merge?I know what the following does, but I don't understand what it does when working with an array:\[code\] $var1 = 1; $var2 = 2; $var1 += $var2; echo $var1; // 3\[/code\]