SimpleEncoder.php

<?php
class SimpleEncoder implements \Iterator
{
    /**
    * @var \Iterator
    */
   private $iterator;
   private $head;
   private $i = 0;

   public function __construct(\Iterator $iterator)
   {
       $this->head = $this->getHeader($iterator->current());
       $this->iterator = $iterator;
   }

   /**
    * Return the current element
    * @link  http://php.net/manual/en/iterator.current.php
    * @return mixed Can return any type.
    * @since 5.0.0
    */
   public function current()
   {
       if ($this->i == 0) {
           return json_encode($this->head) . "\r\n";
       }
       $item = $this->iterator->current();
       return $this->encode($item);
   }

   /**
    * Move forward to next element
    * @link  http://php.net/manual/en/iterator.next.php
    * @return void Any returned value is ignored.
    * @since 5.0.0
    */
   public function next()
   {
       if ($this->i > 0) {
           $this->iterator->next();
       }
       $this->i++;
   }

   /**
    * Return the key of the current element
    * @link  http://php.net/manual/en/iterator.key.php
    * @return mixed scalar on success, or null on failure.
    * @since 5.0.0
    */
   public function key()
   {
       return $this->i;
   }

   /**
    * Checks if current position is valid
    * @link  http://php.net/manual/en/iterator.valid.php
    * @return boolean The return value will be casted to boolean and then evaluated.
    * Returns true on success or false on failure.
    * @since 5.0.0
    */
   public function valid()
   {
       if ($this->i == 0) {
           return true;
       }
       return $this->iterator->valid();
   }

   /**
    * Rewind the Iterator to the first element
    * @link  http://php.net/manual/en/iterator.rewind.php
    * @return void Any returned value is ignored.
    * @since 5.0.0
    */
   public function rewind()
   {
       $this->i = 0;
       $this->iterator->rewind();
   }

   private function getHeader($item)
   {
       if (is_array($item)) {
           ksort($item);
           return array_keys($item);
       }
       return [];
   }

   private function encode($item)
   {
       $line = [];
       foreach ($this->head as $name) {
           if (isset($item[$name])) {
               $line[] = $item[$name];
           }
       }
       return json_encode($line) . "\n";
   }
}