1 “+”：

直接将两个数组的值合并成为一个数组；

$a = [1,2,3,'a'=>'a'];
$b = ['a'=>'b',4,5,'b'=>'c',6,7,'a'];
$c = $a + $b;

print_r($a);
print_r($b);
print_r($c);

//$a
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [a] => a
)

//$b
Array
(
    [a] => b
    [0] => 4
    [1] => 5
    [b] => c
    [2] => 6
    [3] => 7
    [4] => a
)

//$c
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [a] => a
    [b] => c
    [3] => 7
    [4] => a
)

采用"+"合并数组，如果键相同的情况下，前面的数组值会覆盖掉后面的数组值。

2 “array_push”：

将某个变量放入到另一个数组当中

$a = [1,'a'=>'aa',2,'b'=>'bb'];
$b = [1,'a'=>'aa',2,'b'=>'bb'];
print_r($a);
print_r($b);
array_push($a, $b);
print_r($a);

//$a
Array
(
    [0] => 1
    [a] => aa
    [1] => 2
    [b] => bb
)

//$b
Array
(
    [0] => 1
    [a] => aa
    [1] => 2
    [b] => bb
)

// array_push过后的$a
Array
(
    [0] => 1
    [a] => aa
    [1] => 2
    [b] => bb
    [2] => Array
        (
            [0] => 1
            [a] => aa
            [1] => 2
            [b] => bb
        )

)

通过打印的值可以看出，变量进入数组后，对应的数字索引必然是当前数组中最小的数字索引+1（如果当前数组不存在数字索引，新增元素对应的索引为0）

3 “array_merge”：

直接将两个数组进行合并，两个数组键相同的情况，后面数组的值覆盖前面数组的值

$a = [1,'2'=>2,'a'=>'a','b'=>'b'];
$b = [1,'2'=>3,'a'=>'c','b'=>'d','c'=>'e'];
$c = array_merge($a,$b);
print_r($a);
print_r($b);
print_r($c);

//$a
Array
(
    [0] => 1
    [2] => 2
    [a] => a
    [b] => b
)

//$b
Array
(
    [0] => 1
    [2] => 3
    [a] => c
    [b] => d
    [c] => e
)

//$c
Array
(
    [0] => 1
    [1] => 2
    [a] => c
    [b] => d
    [2] => 1
    [3] => 3
    [c] => e
)

可以看出，两个数组键相同的情况，后面数组的值覆盖前面数组的值。但是对于数字索引或者数字字符串索引，会按照顺序进行重置（第一个数组的第一个数字索引元素从0开始依次填充）

4 “array_merge_recursive”：

合并数组，遇到相同的键将该键中值合并为一个子数组

$a = [1,'2'=>2,'a'=>'a','b'=>'b'];
$b = [1,'2'=>3,'a'=>'a','b'=>'d','c'=>'e'];
$c = array_merge_recursive($a,$b);
print_r($a);
print_r($b);
print_r($c);


//$a
Array
(
    [0] => 1
    [2] => 2
    [a] => a
    [b] => b
)

//$b
Array
(
    [0] => 1
    [2] => 3
    [a] => a
    [b] => d
    [c] => e
)

//$c
Array
(
    [0] => 1
    [1] => 2
    [a] => Array
        (
            [0] => a
            [1] => a
        )

    [b] => Array
        (
            [0] => b
            [1] => d
        )

    [2] => 1
    [3] => 3
    [c] => e
)

采用"array_merge_recursive"方法时，会将相同键的元素，整合成为一个新的数组，保持键名不变，作为合并数组（$a与$b组合后的数组）的一个子元素。但是对于数字索引或者数字字符串索引，会按照顺序进行重置（第一个数组的第一个数字索引元素从0开始依次填充）。即使对应的数字索引相同，也不会将相同索引的元素整合成功一个子数组。