Create a valid ICS file to save an event to your calendar.

PHPtoICS.php

<?php
// Variables used in this script:
//   $summary     - text title of the event
//   $datestart   - the starting date (in seconds since unix epoch)
//   $dateend     - the ending date (in seconds since unix epoch)
//   $address     - the event's address
//   $uri         - the URL of the event (add http://)
//   $description - text description of the event
//   $filename    - the name of this file for saving (e.g. my-event-name.ics)
//
// Notes:
//  - the UID should be unique to the event, so in this case I'm just using
//    uniqid to create a uid, but you could do whatever you'd like.
//
//  - iCal requires a date format of "yyyymmddThhiissZ". The "T" and "Z"
//    characters are not placeholders, just plain ol' characters. The "T"
//    character acts as a delimeter between the date (yyyymmdd) and the time
//    (hhiiss), and the "Z" states that the date is in UTC time. Note that if
//    you don't want to use UTC time, you must prepend your date-time values
//    with a TZID property. See RFC 5545 section 3.3.5
//
//  - The Content-Disposition: attachment; header tells the browser to save/open
//    the file. The filename param sets the name of the file, so you could set
//    it as "my-event-name.ics" or something similar.
//
//  - Read up on RFC 5545, the iCalendar specification. There is a lot of helpful
//    info in there, such as formatting rules. There are also many more options
//    to set, including alarms, invitees, busy status, etc.
//
//      https://www.ietf.org/rfc/rfc5545.txt
if( !isset( $_POST['nonce'] ) )
  die();

$summary = $_POST['summary'];
$datestart = $_POST['datestart'];
$dateend = $_POST['dateend'];
$address = $_POST['address'];
$uri = $_POST['uri'];
$description = $_POST['description'];
$filename = $_POST['filename'];
// 1. Set the correct headers for this file
header('Content-type: text/calendar; charset=utf-8');
header('Content-Disposition: attachment; filename=' . $filename);

// 2. Define helper functions

// Converts a unix timestamp to an ics-friendly format
// NOTE: "Z" means that this timestamp is a UTC timestamp. If you need
// to set a locale, remove the "\Z" and modify DTEND, DTSTAMP and DTSTART
// with TZID properties (see RFC 5545 section 3.3.5 for info)
function dateToCal($timestamp) {
	return date('Ymd\THis', $timestamp);
}

// Escapes a string of characters
function escapeString($string) {
	return preg_replace('/([\,;])/','\\\$1', $string);
}

// 3. Echo out the ics file's contents
$br = "\n";
?>
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//hacksw/handcal//NONSGML v1.0//EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
DTSTART:<?php echo dateToCal($datestart) . $br; ?>
DTEND:<?php echo dateToCal($dateend) . $br; ?>
UID:<?php echo uniqid() . $br; ?>
DTSTAMP:<?php echo dateToCal(time()) . $br; ?>
LOCATION:<?php echo escapeString($address) . $br; ?>
DESCRIPTION:<?php echo escapeString($description) . $br; ?>
URL;VALUE=URI:<?php echo escapeString($uri) . $br; ?>
SUMMARY:<?php echo escapeString($summary) . $br; ?>
END:VEVENT
END:VCALENDAR';