AmfPHP 2.0 running on PHP 7 & PDO

Question

AmfPHP 2.0 running on Nginx and PHP 7.0 (FPM), PDO is used as database.

As many users log in to the game, the CPU increases immediately to 100% on all 4 cores due to MySQL. If there are fewer players, the process calms down again and the CPU drops.

Code snippet of PDO connection:

class Database extends PDO {

    public static function getConnection() {
        $connectionString = sprintf("mysql:dbname=%s;host=%s;charset=utf8", MYSQL_DB, MYSQL_HOST);

        try {
            $pdo = new PDO($connectionString, MYSQL_USER, MYSQL_PASS);
            $pdo->setAttribute(\PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION);
            return $pdo;
        } catch(PDOException $e) {
            $error = date("Y-m-d g:i:s a T") . "\Database::getConnection\tError: (" . $e->getCode . ") " . $e->getMessage;
            throw new Exception($error);
        }
    }

    // Example
    public static function Test($arg) {
        try {
            $pdo = Database::getConnection();

            $stmt = $pdo->prepare("select * from xx where xx = :xx");
            $stmt->bindParam(":xx", $arg, PDO::PARAM_STR);
            $stmt->execute();
            $result = $stmt->fetch(PDO::FETCH_ASSOC);

            if ($stmt->rowCount() > 0) {
                // execute my code
            }
        } catch(PDOException $e) {
            $error = date("Y-m-d g:i:s a T") . "\Database::Test\tError: (" . $e->getCode . ") " . $e->getMessage;
            throw new Exception($error);
        }
    }

}

For further questions I am at any time available.