xarxaprod-wp-theme/inc/xarxaprod-ajut-filter.php

<?php
/**
 * Makes a filter for the custom post type ajut and its custom fields
 */

// action
add_action('pre_get_posts', 'my_pre_get_posts');

function my_pre_get_posts( $query ) {
    
    // bail early if is in admin
    if( is_admin() ) { return; }
    

    // allow the url to alter the query
    if( isset($_GET['xxp_fund_target']) )
    {
    // get original meta query
    $meta_query = [];
    $meta_query[] = array('relation'  => 'OR');
    $meta_query[] = $query->get('meta_query');
      $fieldsearchvalues= explode(',', $_GET['xxp_fund_target']);
      foreach( $fieldsearchvalues as $fieldsearchvalue ){
        $os_search_value = $fieldsearchvalue; //'entitat-publica'; // whatever
        $os_field_value = sprintf( '^%1$s$|s:%2$u:"%1$s";', $os_search_value, strlen( $os_search_value ) );

        // add aur meta query to the original meta queries
        $meta_query[] = array(
            array(
                'key'     =>  'xxp_fund_target',
                'value'   =>  $os_field_value,
                'compare' =>  'REGEXP',
            ),
        );
      }
    // update the meta query arguments
    $query->set('meta_query', $meta_query);
    }

    //always return
    return;
}