Foreach 函数在PHP 7 中的变化

阳光穿透心脏的1/2处 2022-06-14 02:19 140阅读 0赞

PHP 7发布至今已有两年多的时间（[7.0.0alpha1][]），相信大部分PHP开发者已经或者正在使用PHP7作为主要开发版本。不过，建议开发者在选择PHP7版本前，仔细认真浏览一下升级日志。作者在实际项目中就遇到了一个`foreach`函数在PHP 5 与PHP 7不同版本下难以发现之坑。

> [PHP 7 UPGRADING][]
> 
> [PHP 7升级备注][PHP 7]

在PHP 5 中执行下面代码：

执行完成后，正常打印结果：

<table style="border-collapse:collapse; border-spacing:0px; margin:0px; position:relative; padding:0px; width:759px"> 
 <tbody> 
  <tr style="margin:0px; padding:0px; width:759px; border-collapse:collapse"> 
   <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(202,202,202); width:10px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 7px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
     
      
     <div style="height:20px">
      
       
      1
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      2
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      3
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      4
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      5
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      6
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      7
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      8
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      9
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      10
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      11
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      12
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      13
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      14
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      15
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      16
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      17
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      18
     
      
     </div></pre> </td> 
   <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(102,102,102); width:726px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 10px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
     
      
     <div style="height:20px"></div>
     
      
     <div style="height:20px">
      
       
      &gt;&gt;&gt; var_dump($stack);
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      <span style="color:rgb(133,153,0)">array</span>(
      
       
      <span style="color:rgb(42,161,152)">7</span>) {
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">0</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"1_0"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">1</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"2_1"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">2</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"3_2"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">3</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"4_3"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">4</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"5_4"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">5</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"6_5"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">6</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"7_6"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      }
     
      
     </div></pre> </td> 
  </tr> 
 </tbody> 
</table>

接着，我们将相同代码放在PHP 7 中运行，结果显示如下：

非常奇怪，为什么打印结果会是一个字符串?

我们继续修改一下源码：

然后在PHP7与PHP5中分别执行，最后都是得到如下结果：

<table style="border-collapse:collapse; border-spacing:0px; margin:0px; position:relative; padding:0px; width:759px"> 
 <tbody> 
  <tr style="margin:0px; padding:0px; width:759px; border-collapse:collapse"> 
   <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(202,202,202); width:10px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 7px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
     
      
     <div style="height:20px">
      
       
      1
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      2
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      3
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      4
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      5
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      6
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      7
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      8
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      9
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      10
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      11
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      12
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      13
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      14
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      15
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      16
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      17
     
      
     </div></pre> </td> 
   <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(102,102,102); width:726px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 10px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
     
      
     <div style="height:20px">
      
       
      &gt;&gt;&gt; var_dump($stack);
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      <span style="color:rgb(133,153,0)">array</span>(
      
       
      <span style="color:rgb(42,161,152)">7</span>) {
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">0</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"1_0"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">1</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"2_1"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">2</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"3_2"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">3</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"4_3"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">4</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"5_4"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">5</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"6_5"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        [
      
       
      <span style="color:rgb(42,161,152)">6</span>]=&gt;
     
      
     </div>
     
      
     <div style="height:20px">
      
       
        string(
      
       
      <span style="color:rgb(42,161,152)">3</span>) 
      
       
      <span style="color:rgb(42,161,152)">"7_6"</span>
     
      
     </div>
     
      
     <div style="height:20px">
      
       
      }
     
      
     </div></pre> </td> 
  </tr> 
 </tbody> 
</table>

继续在PHP5 与 PHP 7中执行如下代码：

PHP 7 中打印结果：

而PHP 5中打印结果：

如果将变量`$stack = [];`赋值未数组，在PHP 5 与PHP 7中都是得到如上结果。

`foreach`函数在PHP 7中的变化

*  foreach() 迭代不再影响数组内部指针，数组指针可通过 current()/next() 等系列的函数访问。例如：
    
    <table style="border-collapse:collapse; border-spacing:0px; margin:0px; position:relative; padding:0px; width:738px"> 
     <tbody> 
      <tr style="margin:0px; padding:0px; width:738px; border-collapse:collapse"> 
       <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(202,202,202); width:10px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 7px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
           
            
         <div style="height:20px">
          
             
            1
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            2
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            3
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            4
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            5
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            6
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            7
           
            
         </div></pre> </td> 
       <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(102,102,102); width:712px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 10px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
           
            
         <div style="height:20px">
          
             
            &gt;&gt;&gt; $array = [
            
             
          <span style="color:rgb(42,161,152)">0</span>, 
            
             
          <span style="color:rgb(42,161,152)">1</span>, 
            
             
          <span style="color:rgb(42,161,152)">2</span>];
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            &gt;&gt;&gt; 
            
             
          <span style="color:rgb(133,153,0)">foreach</span> ($array 
            
             
          <span style="color:rgb(133,153,0)">as</span> &amp;$val) {
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            ...     var_dump(current($array));
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            ... }
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            int(
            
             
          <span style="color:rgb(42,161,152)">0</span>)
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            int(
            
             
          <span style="color:rgb(42,161,152)">0</span>)
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            int(
            
             
          <span style="color:rgb(42,161,152)">0</span>)
           
            
         </div></pre> </td> 
      </tr> 
     </tbody> 
    </table>
    
    现在将指向值 int(0) 三次。以前的输出是 int(1)、int(2) 和 bool(false)。
 *  在对数组按值迭代时，foreach 总是在对数组副本进行操作，在迭代中任何对数组的操作都不会影响到迭代行为。例如：
    
    <table style="border-collapse:collapse; border-spacing:0px; margin:0px; position:relative; padding:0px; width:738px"> 
     <tbody> 
      <tr style="margin:0px; padding:0px; width:738px; border-collapse:collapse"> 
       <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(202,202,202); width:10px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 7px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
           
            
         <div style="height:20px">
          
             
            1
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            2
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            3
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            4
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            5
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            6
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            7
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            8
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            9
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            10
           
            
         </div></pre> </td> 
       <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(102,102,102); width:705px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 10px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
           
            
         <div style="height:20px">
          
             
            $array = [
            
             
          <span style="color:rgb(42,161,152)">0</span>, 
            
             
          <span style="color:rgb(42,161,152)">1</span>, 
            
             
          <span style="color:rgb(42,161,152)">2</span>];
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            $ref =&amp; $array; 
            
             
          <span style="color:rgb(147,161,161)">// Necessary to trigger the old behavior</span>
           
            
         </div>
           
            
         <div style="height:20px">
            
             
          <span style="color:rgb(133,153,0)">foreach</span> ($array 
            
             
          <span style="color:rgb(133,153,0)">as</span> $val) {
           
            
         </div>
           
            
         <div style="height:20px">
          
             
              var_dump($val);
           
            
         </div>
           
            
         <div style="height:20px">  
            
             
          <span style="color:rgb(133,153,0)">unset</span>($array[
            
             
          <span style="color:rgb(42,161,152)">1</span>]);
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            }
           
            
         </div>
           
            
         <div style="height:20px"></div>
           
            
         <div style="height:20px">
          
             
            int(
            
             
          <span style="color:rgb(42,161,152)">0</span>)
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            int(
            
             
          <span style="color:rgb(42,161,152)">1</span>)
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            int(
            
             
          <span style="color:rgb(42,161,152)">2</span>)
           
            
         </div></pre> </td> 
      </tr> 
     </tbody> 
    </table>
    
    现在将打印出全部三个元素 (0 1 2)，而以前第二个元素 1 会跳过 (0 2)。
 *  在对数组按引用迭代时，对数组的修改将继续会影响到迭代。不过，现在 PHP 在使用数字作为键时可以更好的维护数组内的位置。例如，在按引用迭代过程中添加数组元素：
    
    <table style="border-collapse:collapse; border-spacing:0px; margin:0px; position:relative; padding:0px; width:738px"> 
     <tbody> 
      <tr style="margin:0px; padding:0px; width:738px; border-collapse:collapse"> 
       <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(202,202,202); width:10px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 7px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
           
            
         <div style="height:20px">
          
             
            1
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            2
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            3
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            4
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            5
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            6
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            7
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            8
           
            
         </div></pre> </td> 
       <td style="padding:0px; margin:0px; border:1px solid rgb(221,221,221); color:rgb(102,102,102); width:712px; border-collapse:collapse"> <pre style="overflow:auto; font-family:Consolas,Monaco,Menlo,Consolas,monospace; font-size:13px; margin-top:0px; margin-bottom:0px; padding:30px 10px 10px; background:rgb(248,245,236); border:1px solid rgb(221,221,221)">
           
            
         <div style="height:20px">
          
             
            $array = [
            
             
          <span style="color:rgb(42,161,152)">0</span>];
           
            
         </div>
           
            
         <div style="height:20px">
            
             
          <span style="color:rgb(133,153,0)">foreach</span> ($array 
            
             
          <span style="color:rgb(133,153,0)">as</span> &amp;$val) {
           
            
         </div>
           
            
         <div style="height:20px">
          
             
                var_dump($val);
           
            
         </div>
           
            
         <div style="height:20px">
          
             
                $array[
            
             
          <span style="color:rgb(42,161,152)">1</span>] = 
            
             
          <span style="color:rgb(42,161,152)">1</span>;
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            }
           
            
         </div>
           
            
         <div style="height:20px"></div>
           
            
         <div style="height:20px">
          
             
            int(
            
             
          <span style="color:rgb(42,161,152)">0</span>)
           
            
         </div>
           
            
         <div style="height:20px">
          
             
            int(
            
             
          <span style="color:rgb(42,161,152)">1</span>)
           
            
         </div></pre> </td> 
      </tr> 
     </tbody> 
    </table>
    
    现在迭代会正确的添加了元素。如上代码输出是 “int(0) int(1)”，而以前只是 “int(0)”。

来源：https://chenghuiyong.com/

## 参考 ##

*  [https://linux.cn/article-5840-1.html\#4\_4658][https_linux.cn_article-5840-1.html_4_4658]
 *  [PHP’foreach’实际上如何工作？][PHP_foreach]
 *  [https://wiki.php.net/rfc/php7\_foreach][https_wiki.php.net_rfc_php7_foreach]

[7.0.0alpha1]: https://github.com/php/php-src/releases/tag/php-7.0.0alpha1
[PHP 7 UPGRADING]: https://github.com/php/php-src/blob/php-7.0.0beta1/UPGRADING
[PHP 7]: https://linux.cn/article-5840-1.html
[https_linux.cn_article-5840-1.html_4_4658]: https://linux.cn/article-5840-1.html#4_4658
[PHP_foreach]: http://codeday.me/bug/20170304/416.html
[https_wiki.php.net_rfc_php7_foreach]: https://wiki.php.net/rfc/php7_foreach