Effects of __isset() on empty()

gistfile1.php

<?php

// Demonstrating how the presence of __isset() method affects accessing object members

class Person implements ArrayAccess {

	protected $_data = array();
	
	public function __construct($first_name, $last_name)
	{
 		$this->_data['first_name'] = $first_name;
		$this->_data['last_name'] = $last_name;
	}
	
	// Array Access Functions
	
	public function offsetExists($offset)
	{
		return isset($this->_data[$offset]);
	}
	
	public function offsetGet($offset)
	{
		if (isset($this->_data[$offset]))
			return $this->_data[$offset];
		else
			throw new Exception;
	}
	
	public function offsetSet($offset, $value)
	{
		if (isset($this->_data[$offset]))
			$this->_data[$offset] = $value;
		else
			throw new Exception;
	}
	
	public function offsetUnset($offset)
	{
		if (isset($this->_data[$offset]))
			unset($this->_data[$offset]);
		else
			throw new Exception;
	}
	
	// Object Access Functions
	
	/* I am commenting out this function to show how it affects the behavior of the empty() function.
	 * 
	 * public function __isset($offset)
	 * {
	 *		return isset($this->_data[$offset]);
	 * }
	 */
	
	public function __get($offset)
	{
		if (isset($this->_data[$offset]))
			return $this->_data[$offset];
		else
			throw new Exception;
	}
	
	public function __set($offset, $value)
	{
		if (isset($this->_data[$offset]))
			$this->_data[$offset] = $value;
		else
			throw new Exception;
	}

	public function __unset($offset)
	{
		if (isset($this->_data[$offset]))
			unset($this->_data[$offset]);
		else
			throw new Exception;
	}
}

$Person = new Person('Jeremy', 'Lindblom');

echo '<p>get_array: ';
var_dump($Person['first_name']);
echo '</p>';

// This works as expected and empty() returns false.
echo '<p>empty_array: ';
var_dump(empty($Person['first_name']));
echo '</p>';

echo '<p>get_object: ';
var_dump($Person->first_name);
echo '</p>';

// In this case empty() returns true. If we uncomment the __isset() function, it'll return false.
echo '<p>empty_object: ';
var_dump(empty($Person->first_name));
echo '</p>';