Can be found on an Evo blog post with more explanation at the link below.
UPDATE: with PHP8.* this throws an error I haven’t figured out how to satisfactorily resolve.
<form method="post" action="">
<select name="trngregion" id="region" class="postform" onchange="submit();"> <!--establish 'sort value'-->
<option <?php echo ($_POST['trngregion'] == '') ? ' selected="selected"' : ''; ?> value="">All Regions (default)</option>
<option <?php echo ($_POST['trngregion'] == 'Albuquerque') ? ' selected="selected"' : ''; ?> value="Albuquerque">Albuquerque</option>
<option <?php echo ($_POST['trngregion'] == 'Farmington') ? ' selected="selected"' : ''; ?> value="Farmington">Farmington</option>
<option <?php echo ($_POST['trngregion'] == 'Las Cruces') ? ' selected="selected"' : ''; ?> value="Las Cruces">Las Cruces</option>
<option <?php echo ($_POST['trngregion'] == 'Rio Rancho') ? ' selected="selected"' : ''; ?> value="Rio Rancho">Rio Rancho</option>
<option <?php echo ($_POST['trngregion'] == 'Roswell') ? ' selected="selected"' : ''; ?> value="Roswell">Roswell</option>
<option <?php echo ($_POST['trngregion'] == 'Santa Fe') ? ' selected="selected"' : ''; ?> value="Santa Fe">Santa Fe</option>
</select>
</form>
<?php
$paged = ( get_query_var('paged') ) ? get_query_var('paged') : 1;
$today = date('Ymd', strtotime('-6 hours'));
if( !isset($_POST['trngregion']) || '' == $_POST['trngregion']) { //if 'sort value' not selected
$traininglist= new WP_Query( array( //this is the basic query without region selected (the easy part)
'post_type' => 'training',
'posts_per_page' => 6,
'paged' => $paged,
'meta_key' => 'trng_startdate',
'orderby' => 'meta_value',
'order' => 'ASC',
'meta_query' => array(
array(
'key' => 'trng_endate',
'meta-value' => $value,
'value' => $today,
'compare' => '>=',
'type' => 'CHAR'
)
)
) );
} else { <strong>//if sort value exists, the query that compares sort value and custom field value</strong>
$trainingregion = $_POST['trngregion']; //get 'sort value'
$traininglist= new WP_Query( array(
'post_type' => 'training',
'posts_per_page' => -1, //unlimited posts keeps paging from resetting sort
'paged' => $paged,
'meta_key' => 'trng_startdate',
'orderby' => 'meta_value',
'order' => 'ASC',
'meta_query' => array(
'relation' => 'AND', //add query that compares 'sort value' with 'custom field value'
array(
'key' => 'trng_endate',
'meta-value' => $value,
'value' => $today,
'compare' => '>=',
'type' => 'CHAR'
),
array( //add query that compares 'sort value' with 'custom field value'
'key' => 'trng_region', //custom field
'value' => $trainingregion, //sort value
'type' => 'CHAR',
'compare' => '='
)
)
) );
}
if ($traininglist->have_posts()) :
if ($trainingregion) {
echo '<div class="trngfilter"><h2>'.$trainingregion.' Training Events</h2></div>'; //heading for sorted results
}
while ( $traininglist->have_posts() ) : $traininglist->the_post();
?>
Display results here
<?php endwhile; else : echo '<p>There are no upcoming '.$trainingregion.' trainings listed.</p>'; //message for no result endif; wp_pagenavi( array( 'query' => $traininglist ) ); wp_reset_query(); ?>
Reference Links
https://evowebdev.com/2015/02/filtering-custom-post-types-with-dropdown-menu/
