[WP] Generate a downloadable .ics file from any WordPress post or custom post type

wp_ics.php

<?php

/*  
	For a better understanding of ics requirements and time formats
    please check https://gist.github.com/jakebellacera/635416
*/

// UTILS

// Check if string is a timestamp
function isValidTimeStamp($timestamp) {
    //if($timestamp == '') return;
    return ((string) (int) $timestamp === $timestamp) 
        && ($timestamp <= PHP_INT_MAX)
        && ($timestamp >= ~PHP_INT_MAX);
}

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

// Shorten a string to desidered characters lenght - eg. shorter_version($string, 100);
function shorter_version($string, $lenght) {
if (strlen($string) >= $lenght) {
		return substr($string, 0, $lenght);
	} else {
		return $string;
	}
}

// Add a custom endpoint "calendar"
function add_calendar_feed(){
	add_feed('calendar', 'export_ics');
    // Only uncomment these 2 lines the first time you load this script, to update WP rewrite rules, or in case you see a 404
    /*global $wp_rewrite;
    $wp_rewrite->flush_rules( false );*/
}
add_action('init', 'add_calendar_feed');

// Calendar function
function export_ics(){

    // Query the event
    $the_event = new WP_Query(array(
        'p' => $_REQUEST['id'],
        'post_type' => 'any',
    ));
    
    if($the_event->have_posts()) :
        
        while($the_event->have_posts()) : $the_event->the_post();
	
		// If your version of WP < 5.3.0 use the code below

        /*  The correct date format, for ALL dates is date_i18n('Ymd\THis\Z',time(), true)
            So if your date is not in this format, use that function    */
	
        $start_date = date_i18n("Ymd\THis\Z", get_post_meta( get_the_ID(), 'custom-field-of-start-date', true )); // EDIT THIS WITH YOUR OWN VALUE
        $end_date = date_i18n("Ymd\THis\Z", get_post_meta( get_the_ID(), 'custom-field-of-end-date', true )); // EDIT THIS WITH YOUR OWN VALUE
        $deadline = date_i18n("Ymd\THis\Z", get_post_meta( get_the_ID(), 'custom-field-of-deadline-date', true )); // EDIT THIS WITH YOUR OWN VALUE
        
		// Otherwise, if your version of WP >= 5.3.0 use this code
		
		$start_date = get_post_meta( get_the_ID(), 'custom-field-of-start-date', true ); // EDIT THIS WITH YOUR OWN VALUE
        $end_date = get_post_meta( get_the_ID(), 'custom-field-of-end-date', true ); // EDIT THIS WITH YOUR OWN VALUE
        if($start_date != '' && !isValidTimeStamp($start_date)) {
            $start_date = strtotime($start_date);
			$start_date = wp_date("Ymd\THis", $start_date);
        }
        if($end_date != '' && !isValidTimeStamp($end_date)) {
            $end_date = strtotime($end_date);
			$end_date = wp_date('Ymd\THis', $end_date);
		} else {
        	$end_date = wp_date("Ymd\THis", $start_date + (1 * 60 * 60)); // 1 hour after
        }    
		$deadline = date_i18n("Ymd\THis\Z", get_post_meta( get_the_ID(), 'custom-field-of-deadline-date', true )); // EDIT THIS WITH YOUR OWN VALUE
		
		// The rest is the same for any version
		$timestamp = date_i18n('Ymd\THis\Z',time(), true);
		$uid = get_the_ID();
		$created_date = get_post_time('Ymd\THis\Z', true, $uid );
		$organiser = get_bloginfo('name'); // EDIT THIS WITH YOUR OWN VALUE
        $address = ''; // EDIT THIS WITH YOUR OWN VALUE
        $url = get_the_permalink();
        $summary = get_the_excerpt();
        $content = html_entity_decode(trim(preg_replace('/\s\s+/', ' ', get_the_content()))); // removes newlines and double spaces
        $title = html_entity_decode(get_the_title());

        //Give the iCal export a filename
        $filename = urlencode( $title.'-ical-' . date('Y-m-d') . '.ics' );
        $eol = "\r\n";

        //Collect output
        ob_start();

        // Set the correct headers for this file
        header("Content-Description: File Transfer");
        header("Content-Disposition: attachment; filename=".$filename);
        header('Content-type: text/calendar; charset=utf-8');
        header("Pragma: 0");
        header("Expires: 0");

// The below ics structure MUST NOT have spaces before each line
// Credit for the .ics structure goes to https://gist.github.com/jakebellacera/635416
?>
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//<?php echo get_bloginfo('name'); ?> //NONSGML Events //EN
CALSCALE:GREGORIAN
X-WR-CALNAME:<?php echo get_bloginfo('name').$eol;?>
BEGIN:VEVENT
CREATED:<?php echo $created_date.$eol;?>
UID:<?php echo $uid.$eol;?>
DTEND;VALUE=DATE:<?php echo $end_date.$eol; ?>
DTSTART;VALUE=DATE:<?php echo $start_date.$eol; ?>
DTSTAMP:<?php echo $timestamp.$eol; ?>
LOCATION:<?php echo escapeString($address).$eol; ?>
DESCRIPTION:<?php echo $content.$eol; ?>
SUMMARY:<?php echo $title.$eol; ?>
ORGANIZER:<?php echo escapeString($organiser).$eol;?>
URL;VALUE=URI:<?php echo escapeString($url).$eol; ?>
TRANSP:OPAQUE
BEGIN:VALARM
ACTION:DISPLAY
TRIGGER;VALUE=DATE-TIME:<?php echo $deadline.$eol; ?>
DESCRIPTION:Reminder for <?php echo escapeString(get_the_title()); echo $eol; ?>
END:VALARM
END:VEVENT
<?php
        endwhile;
?>
END:VCALENDAR
<?php
        //Collect output and echo
        $eventsical = ob_get_contents();
        ob_end_clean();
        echo $eventsical;
        exit();

    endif;

}
?>

USAGE in TEMPLATE

<a href="<?php echo get_feed_link('calendar'); ?>?id=<?php echo get_the_ID(); ?>"> Download the ics/ical </a>

@Jany-M i have 1 last question i hope the only thing showing in the ics file is the description and i have some problems with the start time end time and the title from the event and as last the address how can i get this to work my meta keys are : _event_title , _event_address , _event_start_date , _event_send_date and as last i want to add the start time and end time to it but dont know how.

i am sorry that i ask so many question but i am kinda new to this so i need some help with it already thanks for helping me so much

@Jany-M Could you please help me out with this were i have to put the meta-keys or something i cant figure it out what i have to do exactly it still doesnt show the title, address, start time and end time

@Jany-M Can you please help me or help me where i can find the exact names of the tables so i can fill them in where you say to fill them in so i can finish this part of the website because i still cant figure out where to find the table names as when i fill in the meta-keys that the plugin says the field doesnt gets filled in when i download the ics file.

@erikvanblitterswijk sorry but I really don't have time at the moment for one on one tutoring on WP or PHP. There are many guides and tutorials out there, besides the WP codex, that will teach you exactly how to get $post title and $post metas.

Oke thanks for telling then i hope i can figure it out what i have to fill in in the 'edit with own value spots to make it work because the its not the meta keys thats for sure and i already looked into the db but there is nothing there about the names so i hope i will figure it out thanks for helping as much as you could

Hi @Jany-M,
A couple of things:

1. It would be nice to document the code a little better. For example, there is no mention that ACF is required where you use get_field(), and it’s especially confusing since in your comments you mention the WP version, which has nothing to do with it (i. e. in the WP < 5.3.0 section you use get_post_meta() and in the WP >= 5.3.0 you use get_field().
2. The function shorter_version() is being defined but never used. You could mention that it’s optional (I suppose for summary and content?).
3. I’m getting an error “Uncaught Error: Call to undefined function isValidTimeStamp()” – yeah, where is that function defined in the first place? Or am I missing anything that the other people before me didn’t?
4. The variable $timestamp you are using on line 108 is defined on line 50 within the section after the WP < 5.3.0 comment, so if you remove that because you’re using a version greater than 5.3.0 an error occurs. What’s the alternative for later versions?
5. The variable $deadline_txt is used on line 90 but never defined before. On the other hand, there is a variable $summary defined on line 56 but never used. Is that simply an oversight?

@VIPStephan

1. I removed ACF get_field, in favor of the standard WP get_post_meta
2. I left it there in case anyone needed it, function's name is pretty self_explanatory, but I've added a better explanation
3. You're right! Added it to script (I had it defined elsewhere)
4. $timestamp should be ok as is, I've moved below it so it's not confusing
5. Correct. In my original script there's more about deadline, which uses ACF true/false and conditional functions in case one is set. I've removed the reference here.

Keep in mind this is a script I used in a real project and added it here mostly for myself, I removed most of project-specific references, it's possible that small parts are not perfect... So, good observations, but remember this isn't a paid php or wp tutorial service, it's just a snippet that other developers can use and manipulate according to need and if they can't figure out the small changes needed, maybe they should hire a dev ;)

On line 66 there is a "g" missing in the function name

On line 66 there is a "g" missing in the function name
Fixed, thank you!