<?php     /**       * Collection that wraps a numeric array.       * All five public methods are needed to implement       * the Iterator interface.       */      class Collection implements Iterator      {   private $_content;   private $_index = 0;   public function __construct(array $content)   {       $this->_content = $content;   }   public function rewind()   {       $this->_index = 0;   }   public function valid()   {       return isset($this->_content[$this->_index]);   }   public function current()   {       return $this->_content[$this->_index];   }   public function key()   {       return $this->_index;   }   public function next()   {       $this->_index++;   }      }      $array = array('A', 'B', 'C', 'D');      echo "Collection: ";      foreach (new Collection($array) as $key => $value) {   echo "$key => $value. ";      }      echo "\n";     /**       * Usually IteratorAggregate is the interface to implement.       * It has only one method, which must return an Iterator       * already defined as another class (e.g. ArrayIterator)       * Iterator gives a finer control over the algorithm,       * because all the hook points of Iterator' contract       * are available for implementation.       */      class NumbersSet implements IteratorAggregate      {   private $_content;   public function __construct(array $content)   {       $this->_content = $content;   }   public function contains($number)   {       return in_array($number, $this->_content);   }   /**    * Only this method is necessary to implement IteratorAggregate.    * @return Iterator    */   public function getIterator()   {       return new ArrayIterator($this->_content);   }      }      echo "NumbersSet: ";      foreach (new NumbersSet($array) as $key => $value) {   echo "$key => $value. ";      }      echo "\n";     // let's play with RecursiveIterator implementations      $it = new RecursiveArrayIterator(array(   'A',   'B',   array(       'C',       'D'   ),   array(       array(    'E',    'F'       ),       array(    'G',    'H',    'I'       )   )      ));      // $it is a RecursiveIterator but also an Iterator,      // so it loops normally over the four elements      // of the array.      echo "Foreach over a RecursiveIterator: ";      foreach ($it as $value) {   echo $value;   // but RecursiveIterators specify additional   // methods to explore children nodes   $children = $it->hasChildren() ? '{Yes}' : '{No}';   echo $children, ' ';      }      echo "\n";      // we can bridge it to a different contract via      // a RecursiveIteratorIterator, whose cryptic name      // should be read as 'an Iterator that spans over      // a RecursiveIterator'.      echo "Foreach over a RecursiveIteratorIterator: ";      foreach (new RecursiveIteratorIterator($it) as $value) {   echo $value;      }      echo "\n";