$meta_query = array();
if (!empty($grad)) {
$meta_query[] = array(
'key' => 'ex_lokacija',
'value' => $grad ,
'compare' => 'LIKE'
)
}
// and so on for each of your keys, then
$args = array(
'post_type' => 'post',
'relation' => 'AND',
'meta_query' => $meta_query
)
$searched_posts = new WP_Query( $args );