foreach 的奇怪行为

这是有据可查的 PHP 行为参见php.net foreach页面上的警告p><块引用>

警告

$value 的引用和最后一个数组元素即使在 foreach 循环之后仍然存在.建议通过 unset() 销毁.

$a = array('a', 'b', 'c', 'd');foreach ($a as &$v) { }未设置($v)；foreach ($a as $v) { }print_r($a);

编辑

尝试了解此处实际发生的情况的分步指南

$a = array('a', 'b', 'c', 'd');foreach ($a as &$v) { }//第一次迭代 $v 是对 $a[0] ('a') 的引用foreach ($a as &$v) { }//第二次迭代 $v 是对 $a[1] ('b') 的引用foreach ($a as &$v) { }//第三次迭代 $v 是对 $a[2] ('c') 的引用foreach ($a as &$v) { }//第 4 次迭代 $v 是对 $a[3] ('d') 的引用//在foreach循环结束时，//$v 仍然是对 $a[3] ('d') 的引用foreach ($a as $v) { }//第一次迭代 $v(仍然是对 $a[3] 的引用)//设置为 $a[0] ('a') 的值.//因为是对$a[3]的引用，//它将 $a[3] 设置为 'a'.foreach ($a as $v) { }//第二次迭代 $v(仍然是对 $a[3] 的引用)//设置为 $a[1] ('b') 的值.//因为是对$a[3]的引用，//它将 $a[3] 设置为 'b'.foreach ($a as $v) { }//第三次迭代 $v(仍然是对 $a[3] 的引用)//设置为 $a[2] ('c') 的值.//因为是对$a[3]的引用，//它将 $a[3] 设置为 'c'.foreach ($a as $v) { }//第 4 次迭代 $v(仍然是对 $a[3] 的引用)//设置为 $a[3] ('c' 因为//最后一次迭代).//因为是对$a[3]的引用，//它将 $a[3] 设置为 'c'.

<?php
  $a = array('a', 'b', 'c', 'd');

  foreach ($a as &$v) { }
  foreach ($a as $v) { }

  print_r($a);
?>

I think it's a normal program but this is the output I am getting:

Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => c
)

Can someone please explain this to me?

This is well-documented PHP behaviour See the warning on the foreach page of php.net

Warning

Reference of a $value and the last array element remain even after the foreach loop. It is recommended to destroy it by unset().

$a = array('a', 'b', 'c', 'd');

foreach ($a as &$v) { }
unset($v);
foreach ($a as $v) { }

print_r($a);

EDIT

Attempt at a step-by-step guide to what is actually happening here

$a = array('a', 'b', 'c', 'd');
foreach ($a as &$v) { }   // 1st iteration $v is a reference to $a[0] ('a')
foreach ($a as &$v) { }   // 2nd iteration $v is a reference to $a[1] ('b')
foreach ($a as &$v) { }   // 3rd iteration $v is a reference to $a[2] ('c')
foreach ($a as &$v) { }   // 4th iteration $v is a reference to $a[3] ('d')

                          // At the end of the foreach loop,
                          //    $v is still a reference to $a[3] ('d')

foreach ($a as $v) { }    // 1st iteration $v (still a reference to $a[3]) 
                          //    is set to a value of $a[0] ('a').
                          //    Because it is a reference to $a[3], 
                          //    it sets $a[3] to 'a'.
foreach ($a as $v) { }    // 2nd iteration $v (still a reference to $a[3]) 
                          //    is set to a value of $a[1] ('b').
                          //    Because it is a reference to $a[3], 
                          //    it sets $a[3] to 'b'.
foreach ($a as $v) { }    // 3rd iteration $v (still a reference to $a[3]) 
                          //    is set to a value of $a[2] ('c').
                          //    Because it is a reference to $a[3], 
                          //    it sets $a[3] to 'c'.
foreach ($a as $v) { }    // 4th iteration $v (still a reference to $a[3]) 
                          //    is set to a value of $a[3] ('c' since 
                          //       the last iteration).
                          //    Because it is a reference to $a[3], 
                          //    it sets $a[3] to 'c'.