如何解决PHP后端功能开发中的并发访问问题？

class DBPool {
    private $connections = [];
    
    public function getConnection() {
        if (empty($this->connections)) {
            $connection = new PDO('mysql:host=localhost;dbname=test', 'root', 'password');
        } else {
            $connection = array_pop($this->connections);
        }
        
        return $connection;
    }
    
    public function releaseConnection($connection) {
        $this->connections[] = $connection;
    }
}

try {
    $pdo->beginTransaction();

    // 执行一系列操作

    $pdo->commit();
} catch (Exception $e) {
    $pdo->rollback();
}

$cacheKey = 'data_key';
$newValue = 'new_value';

$oldValue = getFromCache($cacheKey);

// 判断缓存中的值是否发生变化
if ($oldValue != $newValue) {
    // 如果发生变化，更新缓存
    updateCache($cacheKey, $newValue);
}

$mutex = new Mutex();

if ($mutex->lock()) {
    // 访问共享数据
    $value = getFromCache($cacheKey);
    
    // 释放锁
    $mutex->unlock();
}

$redis = new Redis();
$redis->connect('127.0.0.1', 6379);

// 将请求加入队列
$redis->lpush('request_queue', json_encode($request));

// 从队列中获取请求并处理
while ($request = $redis->rpop('request_queue')) {
    processRequest(json_decode($request, true));
}

$semaphore = new Semaphore(10); // 设置并发请求数为10

if ($semaphore->acquire()) {
    // 处理请求
    
    $semaphore->release();
}