tahvel.edu.ee to Apple and Google calendar

timetable.php

<?php

// Get current timestamp
$now = date("Ymd\THis");

// Retrieve data from tahvel.edu.ee
$data = file_get_contents("https://tahvel.edu.ee/hois_back/timetableevents/timetableByTeacher/15/calendar?from=2019-09-02T00:00:00Z&lang=et&teachers=$_GET[teacher]");

// Convert JSON to PHP array
$data = (array)json_decode($data);

// Create a shortcut to $data['content']
$content = $data['content'];

// Add DTSTAMP property (not really needed, just makes the validator happy)
$content = str_replace("END:VEVENT","DTSTAMP;X-VOBJ-FLOATINGTIME-ALLOWED=TRUE:{$now}\nEND:VEVENT",$content);

// Google Calendar works even without this and validator does not catch that but Apple Calendar breaks
$content = str_replace("VERSION: 2.0","VERSION:2.0",$content);

// Validator complains that lines are not delimited by CRLF sequence
$content = preg_replace('/[\n\r]+/', "\r\n", $content);

// Validator complains that invalid MIME type detected, should be 'text/calendar'
header('Content-Type: text/calendar;charset=UTF-8');

// Output content
echo $content;