Appointment booking feature PHP MySQL schedule available times one year in advance - php

Question

I'm having lots of trouble building an appointment booking site for personal trainers. To understand fully why I'm having so much trouble please be patient and let me explain. I wanted to use this free responsive calendar for the look and feel of my calendar because it's responsive and I like the way it looks, although I'll make some minor alterations to it like color scheme. If you go to the link below you will see a demo of it:
http://tympanus.net/Development/Calendario/index2.html
If you click back on the calendar arrow to November 2012, you will see that certain days have circles on them which indicate that there are events on those days. If you click on the circle a window will come up which is supposed to provide info about the event. The events are fetched from a JSON file whose format is the date for the keys and the values are URLs. When a user clicks on an event instead of the window coming up I want them to be taken to a separate page which will list the available time slots for the trainer on that particular day. Their availability will be based on two things:
1) The schedule that they set for themselves like for instance Mondays, Wednesdays, Fridays from 10am-5pm. I would provide a form on the site where they could set their schedule like this and then save it to a MySQL database and query that database to populate the JSON file.
2) What appointment slots are booked on available days. This is complicated since appointment times can vary. A session can last 15, 30, 45 minutes, 1 hour, 2 hours and 30 minutes, etc. All in 15 minute chunks. So for instance if the session that the user selected is a 1 hour session, I would have to query the schedule of the personal trainer and match that against his appointments booked on every single day and try to figure out where he has 1 hour slots available then show them to the user on a separate page after they've clicked on the calendar date.
I'm having lots of difficulty figuring this out. I can populate the JSON file by querying the schedule of the personal trainer and then using a trick similar to the one below to get all the dates for the next year for a certain day that the trainer is available like for instance Saturday below. Then store that info in the JSON file which will fetch the events for the calendar script:
<?php
$day = 'saturday';
$step = 1;
$unit = 'W';
$start = new DateTime('NOW');
$end = clone $start;
$start->modify($day); // Move to first occurence of day
$end->add(new DateInterval('P1Y')); // Move to 1 year from start
$interval = new DateInterval("P{$step}{$unit}");
$period = new DatePeriod($start, $interval, $end);
foreach ($period as $date) {
echo $date->format('D, d M Y'), PHP_EOL;
echo "<br />";
}
?>

Answer 1

Why would you want to send them to a separate page, when the whole idea of this calendar is to load events inline?
In your json I would only pass the available timeslots for a given day, in order to keep your frontend logic as basic as possible. Figure out serverside what timeslots are still available by walking through that days' events. This is fairly straightforward (sort the appointments, measure the time between the end of event x and the start of event x+1).
Beware of concurrency.
Storing and retrieving schedules and appointments (have you considered recurring appointments too?) can get complicated. You could read up on Martin Fowler, who has an excellent article on modelling schedules and calendars.