#    
## Overview
A **functor** is a functional programming concept that has been adapted from a mathematical field called **category theory**.

In PHP, we can map a function to more than one value contained in the text by using a functor as a design pattern. Moreover, using a map on collections makes them a functor.
## Syntax
Any class that implements the given interface in it is called a functor. 

```PHP
<?php
interface Functor
{
   public function fmap(callable $alpha): Functor;
}
?>
```


## Example
Let's look at the given example, where we implement the functor interface in the `Arr` class.

<?php
class Arr implements Functor
{
    protected $items;

    public function __construct(array $items)
    {
        $this->items = $items;
    }

    public function fmap(callable $alpha): Functor
    {
        return new static(array_map($alpha, $this->items));
    }
}
$add2 = function ($array) { return $array + 1; };
$arr = new Arr([1, 2, 3, 4]);
$mappedArr =$arr->fmap($add2);
print_r($mappedArr)

?>

## Explanation 
- Lines 2–15: We define the class `Arr` that implements the `Functor` interface.
- Line 4: We define the protected data member named `$item`.
- Lines 6–9: We define the constructor of `Arr` class.
- Lines 11–14: We define the `fmap()` function of the `Functor` interface.
- Line 16: We define the anonymous function to add `2` to each index of the `Arr`.
- Line 17: We create an object of the `Arr` class.
- Line 18: We call the `fmap()` function of `Arr` and return the mapped array.

## Explanation 
- Lines 2–15: We define the class `Arr` that implements the `Functor` interface.
- Line 4: We define the protected data member named `＄item`.
- Lines 6–9: We define the constructor of `Arr` class.
- Lines 11–14: We define the `fmap()` function of the `Functor` interface.
- Line 16: We define the anonymous function to add `2` to each index of the `Arr`.
- Line 17: We create an object of the `Arr` class.
- Line 18: We call the `fmap()` function of `Arr` and return the mapped array.

What are functors in PHP?

Functors in PHP allow mapping functions over values within collections, exemplified by classes implementing the Functor interface.