In the header section - Delete action to unlink tickets
<?php
// Delete-Action before the rest of the page is loaded
if (isset($_POST['link_ticket_id']) && hesk_token_check('POST'))   {
    $linked_tickets = explode(",", $ticket['linkedids']);
    $link_ticket_id = $_POST['link_ticket_id'];
    //  Check if posted ID is the ID of this ticket itself
    if ($link_ticket_id == $ticket['id'])   {
        //Message: Ticket is this ticket itself
        hesk_process_messages("Angegeben Ticket-ID ist dieses Ticket selbst.",$_SERVER['PHP_SELF']);
    }
    //  Check if posted ID is allready linked
    elseif (!in_array($link_ticket_id, $linked_tickets)) {
        $res_linking_ticket1 = hesk_dbQuery("SELECT * FROM `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` WHERE `id`='".intval($link_ticket_id)."'");
        //  Check if posted ID exists
        if (hesk_dbNumRows($res_linking_ticket1) != 1)  {
            // Message: Ticket-ID does not exists:
            hesk_process_messages("Angegebene Ticket-ID " . $link_ticket_id . " existiert nicht.",$_SERVER['PHP_SELF']);
        }
        else    {
            // Ticket exists so linkedids-field must be altered:
            $linking_ticket1 = hesk_dbFetchAssoc($res_linking_ticket1);
            $custom_field_links1 = $linking_ticket1['linkedids'].$ticket['id'].',';
            $res_linking_ticket1 = hesk_dbQuery("UPDATE `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` SET `linkedids`='".$custom_field_links1."' WHERE `id`='".intval($link_ticket_id)."'");
            //  Writing posted ID to the Ticket
            $res_linking_ticket2 = hesk_dbQuery("SELECT * FROM `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` WHERE `id`='".intval($ticket['id'])."'");
            $linking_ticket2 = hesk_dbFetchAssoc($res_linking_ticket2);
            $custom_field_links2 = $linking_ticket2['linkedids'].$link_ticket_id.',';
            $res_linking_ticket2 = hesk_dbQuery("UPDATE `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` SET `linkedids`='".$custom_field_links2."' WHERE `id`='".intval($ticket['id'])."'");
            array_push($linked_tickets, $link_ticket_id);
            // Message: Ticket was linked:
            hesk_process_messages("Ticket " . $link_ticket_id . " wurde verknüpft.",$_SERVER['PHP_SELF'],'SUCCESS');
        }
    }
    // Ticket iss allready linked:
    else {
        // Message: Ticket allready linked
        hesk_process_messages("Tickets sind bereits verknüpft.",$_SERVER['PHP_SELF']);
    }
}
if (isset($_GET['del_link']) && hesk_token_check('POST') && $can_delete)   {
    
    //  Linking should be deleted.
    $del_link = $_GET['del_link'];
    $res_del_from_linked_ticket = hesk_dbQuery("SELECT * FROM `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` WHERE `id`='".intval($del_link)."'");
    $del_from_linked_ticket = hesk_dbFetchAssoc($res_del_from_linked_ticket);
    
    //  Ticket-ID will be deleted in the linked Ticket, also the automaticly inserted ','
    $del_string_from_linked_ticket = str_replace($ticket['id'].',', "", $del_from_linked_ticket['linkedids']);
    $res_del_from_linked_ticket = hesk_dbQuery("UPDATE `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` SET `linkedids`='".$del_string_from_linked_ticket."' WHERE `id`='".intval($del_link)."'");

    //  Ticket-Link will be deleted:
    $res_del_linked_ticket = hesk_dbQuery("SELECT * FROM `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` WHERE `id`='".intval($ticket['id'])."'");
    $del_linked_ticket = hesk_dbFetchAssoc($res_del_linked_ticket);
    $del_string_linked_ticket = str_replace($del_link.',', "", $del_linked_ticket['linkedids']);
    $res_del_linked_ticket = hesk_dbQuery("UPDATE `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` SET `linkedids`='".$del_string_linked_ticket."' WHERE `id`='".intval($ticket['id'])."'");

    //Message: Ticket has been deleted:
    hesk_process_messages("Ticket-Verlink wurde gelöscht.",$_SERVER['PHP_SELF'],'SUCCESS');
    exit();

}
?>


    
    <section class="params--block ticketlinks accordion">
        <h4 class="accordion-title">
            <span>Linked Tickets</span>
            <svg class="icon icon-chevron-down">
                <use xlink:href="<?php echo HESK_PATH; ?>img/sprite.svg#icon-chevron-down"></use>
            </svg>
        </h4>
        <div class="accordion-body">
    <?php
        // $ticket['linkedids']; --> new table-column linkedids in the table tickets
        // here all linked tickets are seperated with a ",". So to explode in an array:
        $linked_tickets = explode(",", $ticket['linkedids']);
        //  Checking if there are any tickets linked to be displayed
        if ($ticket['linkedids'] != "" OR $link_ticket_id != "")   {
            foreach ($linked_tickets as $seperated_linked_tickets)   {
                //  Checking if the link was deleted recently, so it should not be displayed anymore
                if (isset($del_link) && $del_link == $seperated_linked_tickets) {
                    // span: Link to display was deleted
                    echo '
                        <div class="row">
                            <span>Verlinkung zu '.$del_link.' entfernt<span>
                        </div>';
                }
                //  Display all linked tickets and the Option to delete
                else    {
                    $res_linked_ticket = hesk_dbQuery("SELECT * FROM `".hesk_dbEscape($hesk_settings['db_pfix'])."tickets` WHERE `id`='".intval($seperated_linked_tickets)."'");
                    $linked_ticket = hesk_dbFetchAssoc($res_linked_ticket);
                    $linked_ticket_trackid = $linked_ticket['trackid'];
                    $linked_ticket_subject = $linked_ticket['subject'];
                    if ($linked_ticket_trackid != "")   {
                        echo '
                        <div class="row">
                            <div style="width: 100%; height: 25px; overflow: hidden;">';
                            //  The option to delete linked tickets is only displayed for those how can delete tickets or have admin-rights
                            if ($can_delete)    {
                                // warning: Should the link be deleted?
                                $warnung = "Soll die Verlinkung wirklich entfernt werden?";
                                echo    '<div style="width:16px; float:right;">
                                    <a class="tooltip" href="admin_ticket.php?track='.$trackingID.'&Refresh='.mt_rand(10000,99999).'&del_link='.$seperated_linked_tickets.'&token='.hesk_token_echo(0).'" onclick="return hesk_confirmExecute(\''.hesk_makeJsString($warnung).'\');" title="Verknüpfung entfernen.">
                                        <svg class="icon icon-delete">
                                            <use xlink:href="'.HESK_PATH.'img/sprite.svg#icon-delete"></use>
                                        </svg>
                                    </a>
                                </div>';
                            }
                            // Checking if the user has the right to see the linked ticket, if not there won't be a link provided
                            if (hesk_okCategory($linked_ticket['category'], 0))   {
                                echo '<a href="?track='.$linked_ticket_trackid.'" class="tooltip" title="$hesklang['subject']: '.$linked_ticket_subject.'">'.$seperated_linked_tickets.' '.$linked_ticket_subject.'</a>';
                            }
                            else    {
                                // title: no right to see this ticket
                                echo '<span class="tooltip" title="Keine Berechtigung zur Einsicht von</br>$hesklang['subject']: '.$linked_ticket_subject.'">'.$seperated_linked_tickets.' '.$linked_ticket_subject.'</span>';
                            }
                            echo '</div>
                        </div>';
                    }
                } 
            }
        }
        //  There haven't been tickets linked yet
        else {
            echo '
                <div class="row">
                    <span>-<span>
                </div>';
        }
    //  At last the ability to link tickets (this could be restricted to specific user-rights like the display of the href-link above)
    ?>               
                <div class="row">
                    <div class="title"><?php echo $hesklang['seqid']; ?>:</div>
                    <div class="value">
                        <form action="?track=<?php echo $trackingID;?>" method="post">
                            <input type="number" name="link_ticket_id" style="width: 70px;" />
                            <input type="hidden" name="track" value="<?php echo $trackingID; ?>">
                            <input type="hidden" name="token" value="<?php if (hesk_token_echo() == "") {hesk_token_echo(0);} else {hesk_token_echo();} ?>" />
                            <input type="submit" name="verknüpfen" value="verknüpfen" style="background-color:white; border: 0px; text-decoration:underline; margin: 4px 2px; color:blue;" />
                        </form>
                    </div>
                </div>
            </div>
        </section>
    

Preview:

Preview of the inserted script