PHP归并两个数组的两种方法的异同

对数组的array_merge函数和+运算符较量疑惑，写了个小措施较量下发明白他们的差异。
出格是+运算符，他的意思是，将右边的数组单位（去一再）追加到左边数组的后头。

<?php
echo "\r\n第一种环境\r\n";
$a=array(1,2,3,4,5,6);
$b=array(7,8,9);

$c=array_merge ($a,$b);
print_r($c);
$c=$a+$b;
print_r($c);
$c=$b+$a;
print_r($c);


echo "\r\n第二种环境\r\n";
$a=array('a','b','c','d','e','f');
$b=array('a','x','y');

$c=array_merge ($a,$b);
print_r($c);
$c=$a+$b;
print_r($c);
$c=$b+$a;
print_r($c);


echo "\r\n第三种环境\r\n";

$a=array(
 1=>'a',
 2=>'b',
 3=>'c',
 4=>'d',
 5=>'e',
 6=>'f');
$b=array(
 1=>'a',
 7=>'x',
 8=>'y');

$c=array_merge ($a,$b);
print_r($c);
$c=$a+$b;
print_r($c);
$c=$b+$a;
print_r($c);
?>

功效如下：

第一种环境
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
    [5] => 6
    [6] => 7
    [7] => 8
    [8] => 9
)
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
    [5] => 6
)
Array
(
    [0] => 7
    [1] => 8
    [2] => 9
    [3] => 4
    [4] => 5
    [5] => 6
)

第二种环境
Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
    [4] => e
    [5] => f
    [6] => a
    [7] => x
    [8] => y
)
Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
    [4] => e
    [5] => f
)
Array
(
    [0] => a
    [1] => x
    [2] => y
    [3] => d
    [4] => e
    [5] => f
)

第三种环境
Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
    [4] => e
    [5] => f
    [6] => a
    [7] => x
    [8] => y
)
Array
(
    [1] => a
    [2] => b
    [3] => c
    [4] => d
    [5] => e
    [6] => f
    [7] => x
    [8] => y
)
Array
(
    [1] => a
    [7] => x
    [8] => y
    [2] => b
    [3] => c
    [4] => d
    [5] => e
    [6] => f
)