[PHP] Call By Reference 的陷阱

class ClassA
{
   private $fields;

   public function test1()
   {
      $this->fields = array(1,2,3);
      foreach ($this->fields as &$v) {
          $v += 1;
      }
                
      $localFields = array('x','y','z');
      foreach ($localFields as $v) {
      }
      echo json_encode($this->fields); // expect: [2,3,4], output: [2,3,"z"]
   }
                
   public function test2()
   {
      $this->fields = array(1,2,3);
      foreach ($this->fields as &$v) {
          $v += 1;
      }

      $v = 'abc';
      echo json_encode($this->fields); // expect: [2,3,4], output: [2,3,"abc"]
   }
                
   public function test3()
   {
      $this->fields = array(1,2,3);
      foreach ($this->fields as &$v) {
          $v += 1;
      }

      $this->inner();
   }

   private function inner()
   {
      $v = '***';
      echo json_encode($this->fields); // output: [2,3,4]
   }
}

$obj = new ClassA();
$obj->test1();
$obj->test2();
$obj->test3();

上面可以看到，雖然第一次使用 &$v 是在迴圈中，第二次使用 $v是在另一個迴圈。 原本預想應該是互不相關，但是顯然 Call By Reference 影響到第一次使用的欄位，造成出乎意料的狀況。

解決的方法：使用完 &$v 之後就馬上 unset($v); 缺點是可能會看不懂為什麼這樣做。

比較好的作法還是改用索引存取：

foreach ($this->fields as $k => $v) {
    $this->fields[$k] = $v + 1;
}

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