操作符+，在前一个数的基础上插入后一个数组，如果有相同的抛弃，而array_merge 是在前一个参数基础上插入后一个参数，如果有相同用的key 用后一个覆盖。实例如下：

php > $a = array('a'=>'1','b'=>'2','c'=>'3');
php > $b = array('c'=>3,'d'=>'4','e'=>'5');
php > $c = $a+$b;
php > $d = array_merge($a,$b);
php > var_dump($c,$d);
array(5) {
  'a' =>
  string(1) "1"
  'b' =>
  string(1) "2"
  'c' =>
  string(1) "3"
  'd' =>
  string(1) "4"
  'e' =>
  string(1) "5"
}
array(5) {
  'a' =>
  string(1) "1"
  'b' =>
  string(1) "2"
  'c' =>
  int(3)
  'd' =>
  string(1) "4"
  'e' =>
  string(1) "5"
}
php > $e = array_merge($b,$a);
php > var_dump($e);
array(5) {
  'c' =>
  string(1) "3"
  'd' =>
  string(1) "4"
  'e' =>
  string(1) "5"
  'a' =>
  string(1) "1"
  'b' =>
  string(1) "2"
}
php > $f=$b+$a;
php > var_dump($f);
array(5) {
  'c' =>
  int(3)
  'd' =>
  string(1) "4"
  'e' =>
  string(1) "5"
  'a' =>
  string(1) "1"
  'b' =>
  string(1) "2"
}

当两边都是索引数组时，使用array_merge 可能会引起索引重置，如下面这种情况：

php > $a = array();
php > $b = array(1=>'data');
php > $c = $a+$b;
php > $d = array_merge($a,$b);
php > var_dump($c,$d);
array(1) {
  [1] =>
  string(4) "data"
}
array(1) {
  [0] =>
  string(4) "data"

}

所以，当使用+来合并两个索引数组，值可能会被丢弃，而array_merge就不会，会把重复的索引重置：

php > $a = array(1=>'one',2=>'two',3=>'three');
php > $b = array(3=>'three',4=>'four',5=>'five');
php > $c = $a+$b;
php > $d = array_merge($a,$b);
php > var_dump($c,$d);
array(5) {
  [1] =>
  string(3) "one"
  [2] =>
  string(3) "two"
  [3] =>
  string(5) "three"
  [4] =>
  string(4) "four"
  [5] =>
  string(4) "five"
}
array(6) {
  [0] =>
  string(3) "one"
  [1] =>
  string(3) "two"
  [2] =>
  string(5) "three"
  [3] =>
  string(5) "three"
  [4] =>
  string(4) "four"
  [5] =>
  string(4) "five"
}

合并两个数组的两种方式的异同