[WordPress] Add file to media library programmatically

gistfile1.php

function upload_image($file_name, $parent_post_id) {
  $file = './img/'.$file_name;
  $filename = basename($file);
  $upload_file = wp_upload_bits($filename, null, file_get_contents($file));
  if (!$upload_file['error']) {
    $wp_filetype = wp_check_filetype($filename, null );
    $attachment = array(
      'post_mime_type' => $wp_filetype['type'],
      'post_parent' => $parent_post_id,
      'post_title' => preg_replace('/\.[^.]+$/', '', $filename),
      'post_content' => '',
      'post_status' => 'inherit'
    );
    $attachment_id = wp_insert_attachment( $attachment, $upload_file['file'], $parent_post_id );
    if (!is_wp_error($attachment_id)) {
      require_once(ABSPATH . "wp-admin" . '/includes/image.php');
      $attachment_data = wp_generate_attachment_metadata( $attachment_id, $upload_file['file'] );
      wp_update_attachment_metadata( $attachment_id,  $attachment_data );
    }

    // SET AS "FEATURED IMAGE"   _\|/_
    set_post_thumbnail($parent_post_id, $attachment_id);
  }
}