Custom query form submission pagination

There are some things that should be changed in your code:

1. Please no not use $_SESSION stuff. So do not add session_start anywhere. You can pass the current state via the query string, using method="get" in your form.
2. For a better flow, you can pass the current page to search_v1 function as argument, in this way you are sure will be not errors
3. When you have to access superglobals array, instead of accessing directly is recomned to use filter_input
4. in the overview-search-input.php file, last line is $query = search_v1($_SESSION['period']); that is completely useless
5. I think is useful in the form select the period option of the current showed posts, e.g. when you are viewing posts for period ‘upcoming’, in the form should be selected the option ‘upcoming’.

Applying this changes:

• page.php and single.php and stay unchanged.
• remove session_start(); from where you have added

Then

overview-search-input.php

<?php
$period = filter_input(INPUT_GET, 'period', FILTER_SANITIZE_STRING);
$paged = get_query_var('paged');
if ( empty($period ) ) $period  = 'upcoming';
if ( empty($paged) ) $paged = 1;

overview-sidebar.php

<form action="<?php echo get_permalink(); ?>" method="get">
<select name="period" placeholder="Select">
  <option <?php selected('', $period) ?>></option>
  <option <?php selected('all', $period) ?>>all</option>
  <option <?php selected('upcoming', $period) ?>>upcoming</option>
</select>
</form>

overview-main.php

include_once 'overview-search-input.php';
include_once 'overview-sidebar.php';

$query = search_v1( $period, $paged );

if( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post();
  // Do stuff
endwhile;

echo create_pagination( $query, $period );

wp_reset_postdata();
endif;

Finally the 2 functions, search_v1 and create_pagination:

function search_v1( $period = 'upcoming', $paged = 1 ) {
  if ( empty($paged) ) $paged = 1;
  $meta_query = array(
    // create your meta query here
  );
  $args = array(
    'post_type'         => 'events',
    'post_status'       => 'publish',
    'posts_per_page'    => 20,
    'paged'             => $paged,
    'meta_query'        => array($meta_query),
    'meta_key'          => 'date_from',
    'orderby'           => 'meta_value',
    'order'             => 'ASC',
  );
  return new WP_Query( $args );
}


function create_pagination( $query, $period = 'upcoming' ) {
  $current = isset($query->query_vars['paged']) ? $query->query_vars['paged'] : 1;
  $args = array(
    'base' => get_permalink() . '%_%',
    'format' => '?paged=%#%&period=' . $period,
    'total' => $query->max_num_pages,
    'current' => ($current > 0) ? $current : 1,
    'show_all' => false,
    'end_size' => 1,
    'mid_size' => 2,
    'prev_next' => true,
    'prev_text' => '<i class="icon-chevron-left"></i>',
    'next_text' => '<i class="icon-chevron-right"></i>',
    'type' => 'list'
  );
  return paginate_links($args);
}