主要區別在與兩個或多個數組在合併時，出現相同的鍵名時處理方式不一樣

一、當鍵名為數字時，array_merge不會覆蓋原來的值，+會拋棄後面的值

<?php
/**
 * Created by PhpStorm.
 * User: lzc
 * Date: 2017/3/18
 * Time: 10:02
 */
$a = [1,2,3];
$b = [4,5,6];
var_dump($a+$b);
var_dump(array_merge($a,$b));
?>

array(3) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
}
array(6) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
  [3]=>
  int(4)
  [4]=>
  int(5)
  [5]=>
  int(6)
}
 

<?php
/**
 * Created by PhpStorm.
 * User: lzc
 * Date: 2017/3/18
 * Time: 10:02
 */
$a = ['a'=>1,'b'=>2,'c'=>3];
$b = ['a'=>4,'d'=>5];
var_dump($a+$b);
var_dump(array_merge($a,$b));
?>

array(4) {
  ["a"]=>
  int(1)
  ["b"]=>
  int(2)
  ["c"]=>
  int(3)
  ["d"]=>
  int(5)
}
array(4) {
  ["a"]=>
  int(4)
  ["b"]=>
  int(2)
  ["c"]=>
  int(3)
  ["d"]=>
  int(5)
}