Web App to iPhone calendar two-way sync
<p>My Web App (PHP) needs to fetch my availability from my iPhone calendar and show which available slots I have in a given time frame. Conversely, those time slots can be booked by the Web App user which should then show up on my iPhone.</p>

<p>Can this be done? Is there an API for this? Or must one go through the extra step of syncing with an external service, and then again: can it be done?</p>