如何使自定义数组类在空时返回布尔值

I have a class as below:

class Fields implements ArrayAccess, Countable, SeekableIterator 
{
   $this->_fields = array();

   ...continue
}

I want to have my class have the following boolean capability

$myArray = new Fields();
if($myArray == false)
{
   echo 'It is empty';
}

How the above behaviour to be implemented in my class

It's not a good solution for your "problem", but it works.

class Fields implements ArrayAccess, Countable, SeekableIterator 
{
  protected $_fields = array();

  // ...continue

  public function __invoke() {
    return $this->_fields == true;
  }
}

I simply implemented an __invoke() method, which gets called if the object gets used as a function. If you now do the following, it works as expected.

$myArray = new Fields();
if($myArray() == false)
{
   echo 'It is empty';
}

This is probably not the best way or the way you should use __invoke, but it's a possibility.