turnstile.php

<?php 

interface TurnstileInterface
{
    public function insertCoin();
    public function pass();
}

class Turnstile implements TurnstileInterface
{
    private $state;
    private $opened;
    private $closed;

    function __construct()
    {
        $this->opened = new TurnstileStateOpened();        
        $this->closed = new TurnstileStateClosed();        
        $this->state  = $this->closed; 
    }

    public function insertCoin()
    {
        $this->state->insertCoin();
        $this->state = $this->opened;
    }

    public function pass()
    {
        $this->state->pass();
        $this->state = $this->closed;
    }
}

class TurnstileStateOpened implements TurnstileInterface
{
    public function insertCoin()
    {
        throw new Exception('Coin already inserted!' . PHP_EOL);
    }

    public function pass()
    {
        echo 'pass! the gate is open!' . PHP_EOL;
    }
}

class TurnstileStateClosed implements TurnstileInterface
{
    public function insertCoin()
    {
        echo 'thanks! opening the gate!' . PHP_EOL;
    }

    public function pass()
    {
        throw new Exception('Alarm! Security!' . PHP_EOL);
    }
}

$turnstile = new Turnstile();
$turnstile->insertCoin();
$turnstile->pass();