WordPress - Custom permalink structure for pages

functions.php

// change page permalink structure

add_action( 'init', 'custom_page_rules' );

function custom_page_rules() {
  global $wp_rewrite;
  $wp_rewrite->page_structure = $wp_rewrite->root . 'my-custom-string/%pagename%'; 
}

add_filter( 'post_link', 'change_page_links', 1, 3 );

function change_page_links( $post_link, $id=0 ){
 
  $post = get_post($id);
 
  if( is_object($post) && $post->post_type == 'page'){
      return home_url('/my-custom-string/'. $post->post_name.'/');
  }

  return $post_link;
}