PHP meeting invite using iCal object, resulting appointment is not editable - php

Question

I'm using a function I found online to generate meeting invites in OutLook using PHP. I have made an online calendar where the users are assigned work orders on a day-to-day basis. I'm trying to "sync" this one with Outlook. Everything works fine, except the resulting Outlook invitation is uneditable. The user must be able to edit the start time and duration of the assignment in Outlook.
The calendar body looks like this:
$ical = 'BEGIN:VCALENDAR' . "\r\n" .
'PRODID:-//Microsoft Corporation//Outlook 10.0 MIMEDIR//EN' . "\r\n" .
'VERSION:2.0' . "\r\n" .
'METHOD:REQUEST' . "\r\n" .
'BEGIN:VTIMEZONE' . "\r\n" .
'TZID:W. Europe Standard Time' . "\r\n" .
'BEGIN:STANDARD' . "\r\n" .
'DTSTART:20181028T030000' . "\r\n" .
'RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10' . "\r\n" .
'TZOFFSETFROM:+0200' . "\r\n" .
'TZOFFSETTO:+0100' . "\r\n" .
'TZNAME:CET' . "\r\n" .
'END:STANDARD' . "\r\n" .
'BEGIN:DAYLIGHT' . "\r\n" .
'DTSTART:20190331T020000' . "\r\n" .
'RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3' . "\r\n" .
'TZOFFSETFROM:+0100' . "\r\n" .
'TZOFFSETTO:+0200' . "\r\n" .
'TZNAME:CEST' . "\r\n" .
'END:DAYLIGHT' . "\r\n" .
'END:VTIMEZONE' . "\r\n" .
'BEGIN:VEVENT' . "\r\n" .
'ORGANIZER;CN="'.$from_name.'":MAILTO:'.$from_address. "\r\n" .
'ATTENDEE;CN="'.$to_name.'";ROLE=REQ-PARTICIPANT;RSVP=TRUE:MAILTO:'.$to_address. "\r\n" .
'LAST-MODIFIED:' . date("Ymd\TGis") . "\r\n" .
'UID:'.date("Ymd\TGis", strtotime($startTime)).rand()."#".$domain."\r\n" .
'DTSTAMP:'.date("Ymd\TGis"). "\r\n" .
'DTSTART:'.date("Ymd\THis", strtotime($startTime)). "\r\n" .
'DTEND:'.date("Ymd\THis", strtotime($endTime)). "\r\n" .
'TRANSP:OPAQUE'. "\r\n" .
'SEQUENCE:1'. "\r\n" .
'SUMMARY:' . $subject . "\r\n" .
'LOCATION:' . $location . "\r\n" .
'CLASS:PUBLIC'. "\r\n" .
'PRIORITY:5'. "\r\n" .
'BEGIN:VALARM' . "\r\n" .
'TRIGGER:-PT15M' . "\r\n" .
'ACTION:DISPLAY' . "\r\n" .
'DESCRIPTION:Reminder' . "\r\n" .
'END:VALARM' . "\r\n" .
'END:VEVENT'. "\r\n" .
'END:VCALENDAR'. "\r\n";
$message .= 'Content-Type: text/calendar;name="meeting.ics";method=REQUEST'."\n";
$message .= "Content-Transfer-Encoding: 8bit\n\n";
$message .= $ical;
$mailsent = mail($to_address, $subject, $message, $headers);
Is there any parameter I can change in order to make the appointment editable?
The organizer is always the same as the attendee in this case.
The headers look like this:
$mime_boundary = "----Meeting Booking----".MD5(TIME());
$headers = "From: ".$from_name." <".$from_address.">\n";
$headers .= "Reply-To: ".$from_name." <".$from_address.">\n";
$headers .= "MIME-Version: 1.0\n";
$headers .= "Content-Type: multipart/alternative; boundary=\"$mime_boundary\"\n";
$headers .= "Content-class: urn:content-classes:calendarmessage\n";

Answer 1

Tony, My understanding is that an ics file that is 'synced' (ie a url that is subscribed to) will not be editable in any application other than the creating one (OR if the receiving application is connected with the creating application, in which case all events will be 'synced' anyway).
If the receiver imports (Ie NOT subscribe to a URL or 'sync') the ics contents into one of their calendars then they should be able to edit it, however it will not 'sync' then.