键名是string，两者区别：
merge 如果键名为字符，且键名相同，array_merge()后面数组元素值会覆盖前面数组元素值
+ 如果键名为字符，且键名相同，数组相加会将最先出现的值作为结果
<?php
$arr1 = array('a'=>'PHP');
$arr2 = array('a'=>'PYTHON');
print_r(array_merge($arr1,$arr2)); //Array ( [a] => PYTHON ) 如果键名为字符，且键名相同，array_merge()后面数组元素值会覆盖前面数组元素值
print_r($arr1+$arr2); //Array ( [a] => PHP )如果键名为字符，且键名相同，数组相加会将最先出现的值作为结果
//再比如
$arr1 = array('a'=>'1','b'=>'2');
$arr2 = array('a'=>'1','b'=>'3','c'=>'2');
$ret = array_merge($arr1,$arr2);
print_r($ret);//Array ( [a] => 1 [b] => 3 [c] => 2 )
$ret = $arr1+$arr2;
print_r($ret);//Array ( [a] => 1 [b] => 2 [c] => 2 )
键名是数字，两者区别：
merge 如果键名为数字，array_merge()不会进行覆盖
+ 如果键名为数字，数组相加会将最先出现的值作为结果，后面键名相同的会被抛弃
<?php
$arr1 = array("JAVA","PHP");
$arr2 = array("PYTHON","PHP");
print_r(array_merge($arr1,$arr2));//Array ( [0] => JAVA [1] => PHP [2] => PYTHON [3] => PHP ) 如果键名为数字，array_merge()不会进行覆盖
print_r($arr1+$arr2);//Array ( [0] => JAVA [1] => PHP )如果键名为数字，数组相加会将最先出现的值作为结果，后面键名相同的会被抛弃
//再比如
$arr1 = array(1=>'1',2=>'2');
$arr2 = array(1=>'1',2=>'3',8=>'2');
$ret = array_merge($arr1,$arr2);
print_r($ret);//Array ( [0] => 1 [1] => 2 [2] => 1 [3] => 3 [4] => 2 ) 如果数组中的键名为数字时，会格式化键名并保留所有键值。
//再比如
$arr1 = array(1,2);
$arr2 = array(1,2,8);
$ret = array_merge($arr1,$arr2);
print_r($ret);//Array ( [0] => 1 [1] => 2 [2] => 1 [3] => 2 [4] => 8 )
?>