如何使用php函数来优化数据库连接速度？

<?php
// 连接到数据库
$mysqli = new mysqli('localhost', 'username', 'password', 'database', null, null, MYSQLI_CLIENT_FOUND_ROWS);

// 检查连接是否成功
if ($mysqli->connect_errno) {
    die('连接数据库失败: ' . $mysqli->connect_error);
}

// 执行查询语句
$result = $mysqli->query('SELECT * FROM users');

// 处理结果集
while ($row = $result->fetch_assoc()) {
    echo $row['username'] . '<br>';
}

// 关闭数据库连接
$mysqli->close();
?>

<?php
class ConnectionPool {
    private static $pool;
    private static $maxConnections = 10;
    private static $currentConnections = 0;
    
    private function __construct() {}
    
    public static function getConnection() {
        if (empty(self::$pool)) {
            self::$pool = new SplQueue();
        }
        
        if (self::$currentConnections < self::$maxConnections) {
            self::$currentConnections++;
            return self::createConnection();
        } else {
            if (!self::$pool->isEmpty()) {
                return self::$pool->dequeue();
            } else {
                return false;
            }
        }
    }
    
    public static function releaseConnection($connection) {
        self::$pool->enqueue($connection);
    }
    
    private static function createConnection() {
        // 创建数据库连接的代码
        $pdo = new PDO('mysql:host=localhost;dbname=database', 'username', 'password');
        return $pdo;
    }
}
?>

<?php
// 连接到Memcached服务器
$memcached = new Memcached();
$memcached->addServer('localhost', 11211);

// 检查缓存中是否存在查询结果
$result = $memcached->get('users');

if (!$result) {
    // 查询数据库并将结果存入缓存
    $mysqli = new mysqli('localhost', 'username', 'password', 'database');
    $result = $mysqli->query('SELECT * FROM users')->fetch_all(MYSQLI_ASSOC);
    $memcached->set('users', $result, 3600); // 将结果缓存1小时
}

// 输出查询结果
foreach ($result as $row) {
    echo $row['username'] . '<br>';
}

// 关闭数据库连接
$mysqli->close();
?>