Fake Booking Object

<?php

namespace BookingNamespace;
/**
* Simulate expiration process with two public methods:
*      - book will return a token
*      - confirm will take a token, and return
*           -> true if called within 5 minutes of corresponding book call
*           -> false if more than 5 minutes have passed
*/
class MyBookingObject
{
    private $tokens;            // array with tokens as keys, dates as values
    private $timeout_delay;     // seconds

    /**
    *   Empty tokens, 5 minutes timeout
    */
    public function __construct()
    {
        $this->tokens = array();
        $this->timeout_delay = 5 * 60;
    }

    private function generate_random_string(){ return substr(md5(rand()), 0, 7); }
    private function has_token($token){ return isset($this->tokens[$token]); }
    private function init_token($token){ $this->tokens[$token] = new DateTime(); }
    private function get_booking_date($token){ return $this->tokens[$token]; }
    private function is_over_timeout($seconds){ return $seconds > $this->timeout_delay; }

    /**
     *  Our booking time has timed out if it has happened too long ago
     *  @return true if $booking_time is too long ago
     *          false if we are on time
     */
    private function has_timed_out(DateTime $booking_time)
    {
        $now = new DateTime();
        if($booking_time > $now) throw new Exception("Booking time should be in the past.");
        $diff = $now->getTimestamp() - $booking_time->getTimestamp();

        return $this->is_over_timeout($diff);
    }

    /**
     * @throws Exception if there is more bookings than the system can handle
     * @return string: the token for our booking.
     */
    public function book()
    {
        if(count($this->tokens) > 10000) throw new Exception("That's too many bookings");
        do {
            $token = $this->generate_random_string();
        }
        while($this->has_token($token));

        $this->init_token($token);
        return $token;
    }

    /**
     * @param string $token: the booking token
     * @throws Exception if this token cannot be found
     * @return boolean true if confirmation ok
     *                 false if confirmation timed out
     */
    public function confirm($token)
    {
        if( ! $this->has_token($token) ) throw new Exception("Incorrect token for confirmation");
        $booking_date = $this->get_booking_date($token);

        if($this->has_timed_out($booking_date)) return false;

        return true;
    }

}
?>