这不是怎么用的add_filter. 通常，您可以使用它将自定义函数附加到某些WP（或插件或主题）过滤器挂钩。然后在自定义函数中进行处理，并让它返回原始挂钩所需的结果类型。

下面是一个如何使用的示例pre_get_avatar 提前检查自定义头像。如果未找到，get_avatar, 在嵌套过滤器的位置，继续执行其常规例程以获取Gravatar图像。这个get_avatar 过滤器挂钩在pre_get_avatar.

add_filter(\'pre_get_avatar\', \'my_pre_get_avatar_filter\', 10, 3);
function my_pre_get_avatar_filter( $avatar, $id_or_email, $args ) {
  $user_id = 0;
  // Can we get an user ID?
  if ( is_int( $id_or_email ) ) {
    $user_id = $id_or_email;
  } else if ( is_email( $id_or_email ) ) {
    $user = get_user_by( \'email\', $id_or_email );
    if ( $user ) {
      $user_id = $user->ID;
    }
  } else if ( is_a( $id_or_email, \'WP_User\' ) ) {
    $user_id = $id_or_email->ID;
  } else {
    return $avatar;
  }
  // If not, bail
  if ( ! $user_id ) {
    return $avatar; // by default this is null
  }
  // Check for custom avatar
  $custom_avatar = get_user_meta( $user_id, \'wp_user_avatar\' );
  if ( is_numeric( $custom_avatar ) && $custom_avatar ) {
    // Here you could just get the attachment url and format the img html yourself to include any available $args
    $custom_avatar = wp_get_attachment_image($custom_avatar, \'thumbnail\');
    // Return custom avatar if image exists
    if ( $custom_avatar ) {
      // this short circuits get_avatar and prevents it from continuing with the default process of checking for Gravatar image, which would be unnecessary as we already have an avatar image
      return $custom_avatar;
    }
  }
  // Return default
  return $avatar;
}