redux.php

<?php

class Store {
    /**
     * @var array
     */
    public $state = [];

    /**
     * @param $reducer
     * @param array $initialState
     * @param $middleware
     */
    public function __construct( $reducer, $initialState = [] ) {
        $this->reducer = $reducer;
        $this->state   = $initialState;
    }

    /**
     * @param $action
     */
    public function dispatch( $action = [] ) {
        $this->state = call_user_func(
            $this->reducer,
            $this->state,
            $action
        );
    }

    /**
     * @return mixed
     */
    public function getState() {
        return $this->state;
    }

    /**
     * @param $cb
     */
    public function subscribe( $cb ) {
        array_push( $this->subscribers, $cb );
        return function () {
            array_splice( $this->subscribers, array_search( $cb, $this->subscribers ), 1 );
        };
    }

}

$store = new Store( function ( $state, $action ) {

    switch ( $action['type'] ) {
    case 'INCREMENT':
        $state['counter'] = $state['counter'] + 1;
        return $state;
    case 'DECREMENT':
        $state['counter'] = $state['counter'] - 1;
        return $state;
    default:
        return $state;
    }

}, [
    'counter' => 0,
] );

$store->dispatch( ['type' => $_GET['action']] );

$state = $store->getState();

var_dump( $state['counter'] );