php数据覆值copy与reference

gistfile1.txt

PHP array assignment (PHP array assignment is copy by value)
$a = array('a', 'b', 'c');
$a2 = $a;
上面的直接覆值是一种copy机制，会产生两个不同的数组供使用

  A duplicate copy of the array (copy by value) is made
  Change in one array have no impact on other
  $a = array("original");
  $b = $a;
  $a[0] = "new";   // $b[0] will remain as "original"

下面引用是指向同一个数组对象，会相互影响。
To copy PHP array by reference
$a3 = &$a;           # A variable alias. Changes impact both $a and $a3


require 'file';//如果文件不存在时会出现错误
include 'file';//如果文件不存在时不会出现错误，只会是警告