Вы можете использовать это:
1)
global $wpdb;
$your_title = "yourtitle";
$id = $wpdb->get_var("SELECT ID FROM $wpdb->posts WHERE post_name = $your_title");
echo $id;
или 2)
$slug_to_get = 'my_title_or_slug';
// you can use custom post type too
$posttypee='post';
$args=array(
'title' => $slug_to_get,
'post_type' => $posttypee,
'post_status' => 'publish'
);
$my_posts = get_posts($args);
if( $my_posts ) {
echo 'ID on the first post found '.$my_posts[0]->ID;
}