PDO connect to a database that currently exists instead of create a new database

I am using PDO class to connect to a sqlite database, here is the code I am using:

public function __construct(){
    parent::__construct("sqlite: userquestion.db");

    $this->exec("PRAGMA foreign_keys = ON;"); // enable foreign keys
}

However, when I instantiate this class, it creates a new database, instead I want to connect to a database that already exists.

PS: yes, I am using the correct directory.