foreach中使用引用所引起的問題

廢話不多說，直接看如下代碼：

<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>

運行後，結果如下：

<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>

我們會詬異，為什麼最後一個是c而不是d？

其實道理很簡單，在第一次循環結束以後，變數v變成了arr[3]的引用。所以，第二次使用foreach時，使用的是傳值的方式，這種方式，其實就是遍歷變數arr的值，然後把值賦給變數v，由於這時候變數v是arr[3]的引用，所以把值賦給變數v，其實就是賦給arr[3]。

我們把程式碼改一下：

<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>

運行後，會發現：

<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>

可以發現，arr[3]的值一直在改變。

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以上就介紹了foreach中使用引用引起的問題，包括了方面的內容，希望對PHP教程有興趣的朋友有所幫助。