PHP开发中如何调试和优化数据库连接

<?php
try {
  $dbh = new PDO('Mysql:host=localhost;dbname=test', 'username', 'passWord');
} catch (PDOException $e) {
  echo '数据库连接失败: ' . $e->getMessage();
}
?>

<?php
$con = mysqli_connect("localhost","username","password","test");

if (mysqli_connect_errno()) {
  echo "数据库连接错误: " . mysqli_connect_error();
}
?>

<?php
try {
  $dbh = new PDO('mysql:host=localhost;dbname=test', 'username', 'password');
} catch (PDOException $e) {
  error_log('数据库连接失败: ' . $e->getMessage(), 3, '/path/to/error.log');
}
?>

<?php
class ConnectionPool {
  private static $pool;

  public static function getInstance() {
    if (!self::$pool) {
      self::$pool = new self();
    }
    return self::$pool;
  }

  private function __construct() {
    // 初始化连接池
  }

  public function getConnection() {
    // 从连接池中获取数据库连接
  }

  public function releaseConnection($connection) {
    // 将连接释放到连接池中
  }
}

$pool = ConnectionPool::getInstance();
$connection = $pool->getConnection();
// 执行数据库操作
$pool->releaseConnection($connection);
?>

<?php
$pdo = new PDO('mysql:host=localhost;dbname=test', 'username', 'password');
$stmt = $pdo->prepare("SELECT * FROM users WHERE age > :age");
$stmt->bindParam(':age', $age, PDO::PARAM_INT);
$stmt->execute();
$result = $stmt->fetchAll(PDO::FETCH_ASSOC);
?>