//看个例子
class My{
 private $_data = [
 'a' => '燕睿涛',
 'b' => 'yanruitao',
 'c' => 'LULU',
 ];
 
 public function getIterator()
 {
 return new ArrayIterator($this->_data);
 }
}
$obj = new My;
foreach ($obj as $key => $value) {
 echo "$key => $value\n";
}
//输出结果为空 

class My implements IteratorAggregate {
 private $_data = [
 'a' => '燕睿涛',
 'b' => 'yanruitao',
 'c' => 'LULU',
 ];

 public function getIterator()
 {
 return new ArrayIterator($this->_data);
 }
}
$obj = new My;
foreach ($obj as $key => $value) {
 echo "$key => $value\n";
}
//结果：
a => 燕睿涛
b => yanruitao
c => LULU

class CountMe
{ 
 protected $_myCount = 3; 

 public function count() 
 { 
 return $this->_myCount; 
 } 
} 

$countable = new CountMe(); 
echo count($countable);
//返回1

class CountMe implements Countable
{ 
 protected $_myCount = 3; 

 public function count() 
 { 
 return $this->_myCount; 
 } 
} 

$countable = new CountMe(); 
echo count($countable); 
//返回3


ArrayAccess接口
ArrayAccess {
 abstract public boolean offsetExists(mixed $offset)
 abstract public mixed offsetGet(mixed $offset)
 public void offsetSet(mixed $offset, mixed $value)
 public void offsetUnset(mixed $offset)
}


class CountMe
{ 
 protected $_myCount = 3; 

 public function count() 
 { 
  return $this->_myCount; 
 } 
} 

$countable = new CountMe(); 
echo count($countable);
//返回1

class CountMe implements Countable
{ 
 protected $_myCount = 3; 

 public function count() 
 { 
  return $this->_myCount; 
 } 
} 

$countable = new CountMe(); 
echo count($countable); 
//返回3

class myObj
{
 
}
$obj = new myObj;
$obj['name'];
//Fatal error: Cannot use object of type myObj as array in 

class myObj implements ArrayAccess 
{
 public function offsetSet($offset, $value)
 {
  echo "offsetSet : {$offset} => {$value}\n";
 }

 public function offsetExists($offset)
 {
  echo "offsetExists : {$offset}\n";
 }

 public function offsetUnset($offset)
 {
  echo "offsetUnset : {$offset}\n";
 }

 public function offsetGet($offset)
 {
  echo "offsetGet : {$offset}\n";
 }
}
$obj = new myObj;
$obj[1] = '燕睿涛';
isset($obj['name']);
unset($obj['name']);
$obj['yrt'];

//输出结果：
offsetSet : 1 => 燕睿涛
offsetExists : name
offsetUnset : name
offsetGet : yrt

class myObj implements ArrayAccess 
{
 private $_data = [];
 public function offsetSet($offset, $value)
 {
  $this->_data[$offset] = $value;
 }

 public function offsetExists($offset)
 {
  return isset($this->_data[$offset]);
 }

 public function offsetUnset($offset)
 {
  unset($this->_data[$offset]);
 }

 public function offsetGet($offset)
 {
  return $this->_data[$offset];
 }
}

$obj = new myObj;
$obj['yrt'] = '燕睿涛';
var_dump($obj['yrt']);
var_dump(isset($obj['yrt']));
unset($obj['yrt']);
var_dump(isset($obj['yrt']));
var_dump($obj['yrt']);

//输出：
string(9) "燕睿涛"
bool(true)
bool(false)
Notice: Undefined index: yrt //最后一个会报出Notice

class myObj implements ArrayAccess, IteratorAggregate
{
private $_data = [];

 public function getIterator()
 {
  return new ArrayIterator($this->_data);
 }

 ......
}
$obj = new myObj;
$obj['yrt'] = '燕睿涛';
$obj[1] = '燕睿涛';
$obj['name'] = '燕睿涛';
$obj['age'] = 23;

foreach ($obj as $key => $value) {
 echo "{$key} => {$value}\n";
}
//输出：
yrt => 燕睿涛
1 => 燕睿涛
name => 燕睿涛
age => 23

class myObj implements Iterator{

 private $_data = [];

 public function __construct(Array $arr)
 {
 $this->_data = $arr;
 }

 public function current()
 {
 return current($this->_data);
 }

 public function key()
 {
 return key($this->_data);
 }

 public function next()
 {
 next($this->_data);
 }

 public function rewind()
 {
 reset($this->_data);
 }

 public function valid()
 {
 return $this->key() !== NULL;
 }
}

$t = [
 'yrt' => '燕睿涛',
 'name' => '燕睿涛',
 false,
 '燕睿涛'
];
$obj = new myObj($t);

foreach ($obj as $key => $value) {
 echo "{$key} => ".var_export($value, true)."\n";
}
//输出：
yrt => '燕睿涛'
name => '燕睿涛'
0 => false
1 => '燕睿涛'