如何正确理解php数组合并函数 array_merge() ?

$a = ["苏"];
$b = ["南"];
$c = ["大","叔"];
$x = array_merge($a,$b,$c);
print_r($x);
// Array ( [0] => 苏 [1] => 南 [2] => 大 [3] => 叔 )

$a = array(0 => "苏", 1 => "南");
$b = array(0 => "su");
$x = array_merge($a, $b);
print_r($x);

Array
(
    [0] => 苏
    [1] => 南
    [2] => su
)

$a = array("a" => "苏", "b" => "南");
$b = array("a" => "su");
$x = array_merge($a, $b);
print_r($x);

Array
(
    [a] => su
    [b] => 南
)

$a = array("0" => "苏", "b" => "南");
$b = array("b" => "su", 0 => "nan", "0" => "大叔");
$x = array_merge($a, $b);
print_r($x);

Array ( [0] => 苏 [b] => su [1] => 大叔 )