PHP 实现知识问答网站中的用户关注功能。

CREATE TABLE `follows` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `follower_id` int(11) unsigned NOT NULL,
  `following_id` int(11) unsigned NOT NULL,
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  UNIQUE KEY `follower_following_unique` (`follower_id`,`following_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

<?php
// 获取当前用户的 ID，可以从登录信息中获取。
$currentUser = 1;

// 获取被关注用户的 ID，可以从用户界面中获取。
$followingUser = 2;

// 连接数据库
$pdo = new PDO('mysql:host=localhost;dbname=test', 'username', 'password');

// 检查是否已经关注
$sql = "SELECT COUNT(*) FROM follows WHERE follower_id = :follower AND following_id = :following";
$query = $pdo->prepare($sql);
$query->bindParam(':follower', $currentUser);
$query->bindParam(':following', $followingUser);
$query->execute();
$alreadyFollowing = $query->fetchColumn() > 0;

if ($alreadyFollowing) {
  // 如果已经关注，则取消关注
  $sql = "DELETE FROM follows WHERE follower_id = :follower AND following_id = :following";
} else {
  // 如果未关注，则添加关注
  $sql = "INSERT INTO follows (follower_id, following_id) VALUES (:follower, :following)";
}

$query = $pdo->prepare($sql);
$query->bindParam(':follower', $currentUser);
$query->bindParam(':following', $followingUser);
$query->execute();

// 关闭数据库连接
$pdo = null;
?>