foreach中使用引用引起的问题

废话不多说，直接看如下代码：

<code><span>$arr</span> = <span>array</span>(<span>'a'</span>,<span>'b'</span>,<span>'c'</span>,<span>'d'</span>);
     <span>foreach</span>(<span>$arr</span><span>as</span> &<span>$v</span>){}

     <span>foreach</span>(<span>$arr</span><span>as</span><span>$v</span>){
          var_dump(<span>$v</span>);
     }</code>

运行后，结果如下：

<code><span>string</span><span>'a'</span> (<span>length</span>=<span>1</span>)
<span>string</span><span>'b'</span> (<span>length</span>=<span>1</span>)
<span>string</span><span>'c'</span> (<span>length</span>=<span>1</span>)
<span>string</span><span>'c'</span> (<span>length</span>=<span>1</span>)    <span> //这里为什么是c呢？</span></code>

我们会诧异，为什么最后一个是c而不是d？

其实道理很简单，在第一次循环结束以后，变量v变成了arr[3]的引用。所以，当第二次使用foreach时，使用的是传值的方式，这种方式，其实就是遍历变量arr的值，然后把值赋给变量v，由于这时候变量v是arr[3]的引用，所以把值赋给变量v，其实就是赋给arr[3]。

我们把代码改一下：

<code><span>$arr</span> = <span>array</span>(<span>'a'</span>,<span>'b'</span>,<span>'c'</span>,<span>'d'</span>);
     <span>foreach</span>(<span>$arr</span><span>as</span> &<span>$v</span>){}

     <span>foreach</span>(<span>$arr</span><span>as</span><span>$v</span>){
          var_dump(<span>$arr</span>);<span>//这里把变量$arr打印出来</span>
     }</code>

运行后，会发现：

<code>array (size=<span>4</span>)
  <span>0</span> => <span>string</span><span>'a'</span> (<span>length</span>=<span>1</span>)
  <span>1</span> => <span>string</span><span>'b'</span> (<span>length</span>=<span>1</span>)
  <span>2</span> => <span>string</span><span>'c'</span> (<span>length</span>=<span>1</span>)
  <span>3</span> => &<span>string</span><span>'a'</span> (<span>length</span>=<span>1</span>)
array (size=<span>4</span>)
  <span>0</span> => <span>string</span><span>'a'</span> (<span>length</span>=<span>1</span>)
  <span>1</span> => <span>string</span><span>'b'</span> (<span>length</span>=<span>1</span>)
  <span>2</span> => <span>string</span><span>'c'</span> (<span>length</span>=<span>1</span>)
  <span>3</span> => &<span>string</span><span>'b'</span> (<span>length</span>=<span>1</span>)
array (size=<span>4</span>)
  <span>0</span> => <span>string</span><span>'a'</span> (<span>length</span>=<span>1</span>)
  <span>1</span> => <span>string</span><span>'b'</span> (<span>length</span>=<span>1</span>)
  <span>2</span> => <span>string</span><span>'c'</span> (<span>length</span>=<span>1</span>)
  <span>3</span> => &<span>string</span><span>'c'</span> (<span>length</span>=<span>1</span>)
array (size=<span>4</span>)
  <span>0</span> => <span>string</span><span>'a'</span> (<span>length</span>=<span>1</span>)
  <span>1</span> => <span>string</span><span>'b'</span> (<span>length</span>=<span>1</span>)
  <span>2</span> => <span>string</span><span>'c'</span> (<span>length</span>=<span>1</span>)
  <span>3</span> => &<span>string</span><span>'c'</span> (<span>length</span>=<span>1</span>)</code>

可以发现，arr[3]的值一直在改变。

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以上就介绍了foreach中使用引用引起的问题，包括了方面的内容，希望对PHP教程有兴趣的朋友有所帮助。