Change the exsisting $_GET to increment by 1

I want to change my URL only on 'Overnight' to increase by 1 each time its clicked if it is already ?ago=1 I want ?ago=2 as the href and if ?ago=2 increment to ?ago=3 etc

I've try to do it with function filter_overnight but it's not working?

<!-- below is the html list items -->


<li id="menu-item-65831"><a href="?<?php echo $twd_helper->query_string(array('tod' => 'morning'), array('page'))  ?>">Morning</a></li>
<li id="menu-item-65832"><a href="?<?php echo $twd_helper->query_string(array('tod' => 'afternoon'), array('page')) ?>">Afternoon</a></li>
<li id="menu-item-65833"><a href="?<?php echo $twd_helper->query_string(array('tod' => 'evening'), array('page')) ?>">Evening</a></li>
<li id="menu-item-65834"><a href="?<?php echo $twd_helper->query_string(array('tod' => 'overnight'), array('page'), array('ago')) ?>">Overnight</a></li>


<!-- below is the included common php routines -->

<?php

function filter_where($where = '') {

    // posts in the last 30 days
    if (isset($_GET['ago'])) {

        if (is_page('future-tv-guide')) {
            $week = date('Y-m-d');
        } else {
            $week = date('Y-m-d', strtotime('-7 days'));
        }
        if (is_page('future-tv-guide')) {
            $previous = intval($_GET['ago']);
            $specific_date = date('Y-m-d', strtotime('+' . $previous . ' days'));
        } else {
            $previous = intval($_GET['ago']);
            $specific_date = date('Y-m-d', strtotime('-' . $previous . ' days'));
        }
        if ($_GET['ago'] == "week") {
            $where .= " AND post_date > '" . $week . "'";
        } else {
            $where .= " AND post_date>='$specific_date 00:00:00' AND post_date <= '$specific_date 23:59:59'";
        }
    }

    return $wher开发者_如何学编程e;

}



class TimeForTellyWPHelper {
    public function __construct($future=false) {
        $GLOBALS['twd_helper']=&$this;
    }   
    public function query_string($add, $remove=null) {
        $qs_data = array();
        parse_str($_SERVER['QUERY_STRING'], $qs_data);
        $qs_data = array_merge($qs_data, $add);
        if ($remove) {
            foreach($remove as $key) {
                if (isset($qs_data[$key])) {
                    unset($qs_data[$key]);
                }
            }
        }
        return http_build_query($qs_data);
    }

    public function filter_overnight () {
        if (isset($_GET['tod']) && $_GET['tod'] == 'overnight') {
            if ($_GET['ago'])
            {
                $_GET['ago'] = (int)$_GET['ago'] + 1;
            }
            else
            {
                $_GET['ago'] = 0;
            }
        }

    }   

    public function get_filter_summary() {
        if (isset($_GET['category']) && $_GET['category'] != 'all') {
            $category = $_GET['category'];
        }
        if (isset($_GET['channel']) && $_GET['channel'] != 'all') {
            $channel = $_GET['channel'];
        }
        if (isset($_GET['ago']) && $_GET['ago'] != 'week') {
            $ago = $_GET['ago'];

        }

        $summary = 'Results: ';
        if ($category || $ago || $channel) {
            if ($category) {
                $summary .= get_term_by('slug', $category, 'programmecategories')->name . "";
                } else {
                    $summary .= "All Categories";
                }

            if ($ago) {
                if (is_page('future-tv-guide')) {
                    if ($ago == "week") {
                        $summary .= " next week.";
                        } elseif ($ago == "1") {

                            $summary .= " / Tomorrow";

                        } else {

                        $thedate  = mktime(0, 0, 0, date('m'), date('d') + $ago, date('Y'));

                            //$summary .= " / in " . $ago . " days time";
                            $summary .= " / " . date( 'l jS', $thedate );
                    }
                } else {

                    if ($ago == "week") {

                        $summary .= " last week.";

                        } elseif ($ago == "1") {

                            $summary .= " / Yesterday.";

                        } else {

                        $thedate  = mktime(0, 0, 0, date('m'), date('d') - $ago, date('Y'));

                            //$summary .= " / in " . $ago . " days time";
                            $summary .= " / " . date( 'l jS', $thedate );
                    }
                }
            } else {
                if (is_page('future-tv-guide')) {
                    $summary .= " / Today";
                } else {
                    $summary .= " / Last week";
                }

            }

            if ($channel) {
                $summary .= " / " . get_term_by('slug', $channel, 'channelnames')->name . "";
                } else {
                    $summary .= " / All Channels";
                }

            return $summary;
        }
        return '';
    }



    public function get_posts($past) {

        $page = isset($_GET['page']) ? intval($_GET['page']) : 1;
        $posts_per_page = isset($_GET['per_page']) ? intval($_GET['per_page']) : 10;

        $query = array(
            'post_type' => 'programmes',
            'posts_per_page' => $posts_per_page,
            'paged' => $page,
            'suppress_filters' => false
        );

        if (!$past) {
            $query['orderby'] = 'date';
            $query['order'] = 'ASC';
        } else {

            if (isset($_GET['sort']) && $_GET['sort'] == 'recent') {
                $query['orderby'] = 'date';
                $query['order'] = 'DESC';
            } else {
                $query['order'] = 'DESC';
                $query['orderby'] = 'meta_value_num';
                $query['meta_key']= 'popularityfig';
            }
        }

        $tax_query = array(
            'relation' => 'AND'
        );
        $meta_query = array();
        if ($past) {
            $query['post_status'] = 'publish';
            $meta_query[] = array(
                'key' => 'linktovideocatchup',
                'value' => "",
                'compare' => 'NOT IN'
            );
        } else {
            $query['post_status'] = 'future';
        }

        if (isset($_GET['category'])) {
            if ($_GET['category'] != 'all')
                $tax_query[] = array(
                    'taxonomy' => 'programmecategories',
                    'field' => 'slug',
                    'terms' => array( $_GET['category'] ),
                );
        }

        if (isset($_GET['channel'])) {
            if ($_GET['channel'] != 'all')
                $tax_query[] = array(
                    'taxonomy' => 'channelnames',
                    'field' => 'slug',
                    'terms' => array( $_GET['channel'] ),
                );
        }
        if (isset($_GET['search'])) {
            $meta_query[] = array(
                'key' => 'Programme Name',
                'value' => $_GET['search'],
                'compare' => 'LIKE'
            );
        }
        if (isset($_GET['tod'])) {
                $meta_query[] = array(
                    'key' => 'Programme TOD',
                    'value' => $_GET['tod'],
                    'compare' => 'EXACT'
                );
        }
        if (isset($_GET['letter'])) {
            $meta_query[] = array(
                'key' => 'Programme Prefix',
                'value' => $_GET['letter'],
                'compare' => 'EXACT'
            );
        }
        if (sizeof($tax_query) > 1) {
            $query['tax_query'] = $tax_query;
        }
        if (sizeof($meta_query) > 0) {
            $query['meta_query'] = $meta_query;
        }
        add_filter( 'posts_where', 'filter_where' ); 
        return get_posts($query);
    }
}
?>

You're code's really long, please remove the unnecessary areas. As for your question, what you should be doing is this (only an example, you'll need to adapt it to your framework)

PHP

$ago = isset($_GET['ago']) ? (int) $_GET['ago'] : 0; //initally 0 on page load
//do some stuff with ago
$ago++; //ago is now +1 

HTML

<a href='/your-url/<?php echo $ago?'>Ago</a>

So how it works is, initially you'll have $ago at 0 because it hasn't been set. But when the HTML is generated, the link will be pointing to your page with $ago + 1 since we incremented it at the bottom