A simple class that wraps a PHP array in a "functional" way

FunctionalArray.php

<?php

/**
 * A simple class that wraps a PHP array in a functional way.
 *
 * Example Usage:
 *
 *   $val = (new FunctionalArray(range(1, 10)))
 *     ->map    (function($value)      { return $value * 1;   })
 *     ->filter (function($value)      { return $value > 5;   })
 *     ->reduce (function($acc, $curr) { return $acc + $curr; }, 0);
 *
 *   print_r($val);
 */

class FunctionalArray implements ArrayAccess, Iterator, JsonSerializable {
  private $array = [];
  private $keys = [];
  private $position = null;
  
  public function __construct($array = []) {
    $this->array = $array;
    $this->keys = array_keys($array);
    $this->position = array_key_exists(0, $this->keys)
        ? $this->keys[0]
        : null;
  }
  
  public function map($callback) {
    $new = [];
    foreach($this->array as $key => $value) {
      $new[$key] = $callback($value, $key);
    }
    return new FunctionalArray($new);
  }
  
  public function filter($callback) {
    $result = array_filter($this->array, $callback, ARRAY_FILTER_USE_BOTH);
    return new FunctionalArray(
      self::has_string_keys($this->array)
        ? $result
        : array_values($result)
    );
  }
  
  public function reduce($callback, $initialValue = NULL) {
    $accumulator = $initialValue;
    foreach($this->array as $key => $value) {
      $accumulator = $callback($accumulator, $value, $key);
    }

    return is_array($accumulator)
      ? new FunctionalArray($accumulator)
      : $accumulator;
  }

  public function get() {
    return $this->array;
  }

  public function offsetExists($offset) {
    return array_key_exists($offset, $this->array);
  }
  
  public function offsetGet($offset) {
    return $this->array[$offset];
  }
  
  public function offsetSet($offset, $value) {
    $this->array[$offset] = $value;
  }
  
  public function offsetUnset($offset) {
    unset($this->array[$offset]);
  }

  public function rewind() {
    $this->position = 0;
  }

  public function current() {
    return $this->array[
      $this->keys[$this->position]
    ];
  }

  public function key() {
    return $this->keys[$this->position];
  }

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

  public function valid() {
    return 
      isset($this->keys[$this->position]) && 
      isset($this->array[
        $this->keys[$this->position]      
      ]);
  }

  public function jsonSerialize() {
    return json_encode($this->array);
  }

  private static function has_string_keys(array $array) {
    return count(array_filter(array_keys($array), 'is_string')) > 0;
  }
}

FunctionalArrayExamples.php

<?php

include 'FunctionalArray.php';

#######################################
## Map/Filter/Reduce using numeric keys
#######################################
$factor = 2;
$numbers = (new FunctionalArray(range(1, 10)))
  ->map    (function($value) use ($factor)  { return $value * $factor;  })
  ->filter (function($value)                { return $value >= 10;      });
print_r($numbers->get());

## Output: 
## Array
## (
##     [0] => 10
##     [1] => 12
##     [2] => 14
##     [3] => 16
##     [4] => 18
##     [5] => 20
## )




###########################################
## Map/Filter/Reduce using associative keys
###########################################
$fruitRatings = [
  'apple' => 70, 
  'banana' => 80, 
  'cherry' => 90, 
  'dragonfruit' => 100
];
$favoriteFruits = (new FunctionalArray($fruitRatings))
  ->filter(function($value)     { return $value >= 80; })
  ->map(function($value, $key)  { return "($value)"; })
  ->reduce(function($acc, $value, $key) {
    return implode("\n", [$acc, "$key $value"]);
  }, "My favorite fruits:");
echo "\n" . $favoriteFruits;

## Output: 
## My favorite fruits:
## banana (80)
## cherry (90)
## dragonfruit (100)




#########################
## Implements ArrayAccess
#########################
$names = new FunctionalArray([
  'Alex', 
  'Bailey',
  'Cameron',
  'Darcy',
  'Erin',
]);
echo "\n\nNames: ";
print_r($names->get());
echo '$names[0]: ' . "$names[0] \n";
echo '$names[4]: ' . "$names[4] \n";

## Output: 
## Names: Array
## (
##     [0] => Alex
##     [1] => Bailey
##     [2] => Cameron
##     [3] => Darcy
##     [4] => Erin
## )
## $names[0]: Alex 
## $names[4]: Erin 




######################
## Implements Iterator
######################
$names = new FunctionalArray([
  'a' => 'Alpha',
  'b' => 'Bravo',
  'c' => 'Charlie',
  'd' => 'Delta',
  'e' => 'Echo', 
  'f' => 'Foxtrot',
]);
echo "\n\nNATO Names: \n";
foreach($names as $letter => $name) {
  echo "$letter: $name \n";
}

## Output:
## NATO Names: 
## a: Alpha 
## b: Bravo 
## c: Charlie 
## d: Delta 
## e: Echo 
## f: Foxtrot 




##############################
## Implements JsonSerializable
##############################
echo "\nSerialize as JSON: ";
$data = new FunctionalArray([
  'a' => 1,
  'b' => 2,
]);
echo json_encode($data);

## Output:
## Serialize as JSON: "{\"a\":1,\"b\":2}"