PHP pdo异常“找不到驱动程序” pgSQL的

所以我总是得到一个pdo异常，当我尝试在我的Mapper类的构造函数中创建我的数据库。
在这一行：

So i always get a pdo exception when i try to create my db in the constructor of my Mapper class. on this line:

$this->db = new PDO($dsn, $db_config['username'], $db_config['password']);

这是我的dsn创作：

    $db_config = array(
    'driver' => 'pgsql',
    'username' => $dbUser,
    'password' => $dbPassword,
    'schema' => 'r0628740',
    'dsn' => array(
        'host' => 'gegevensbanken.khleuven.be',
        'dbname' => '2TX31',
        'port' => '51314',
    )
);

并完成我的构造函数：

public function __construct(){
        global $db_config;
        $dsn = $db_config['driver'] . ':';
        foreach($db_config['dsn'] as $key => $value){
            $dsn .= $key . '=' . $value . ';';
        }
        try{

            $this->db = new PDO($dsn, $db_config['username'], $db_config['password']);
            $this->db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

            if(($db_config['driver'] == 'pgsql') && isset($db_config['schema'])){
                $this->db->query(sprintf("SET SEARCH_PATH TO %s"), $db_config['schema']);
            }
        }catch (PDOException $e){
            var_dump($e->getLine());
            error_log($e->getMessage());
        }

    }