signup class:

function __construct() {
	require 'db.php';
	$this->db = new db();
}
    public function checkDuplicate($email) {
        $this->db->connect();
        $q = $this->db->conn->query("SELECT email FROM table WHERE email = $email");

        return (($q->rowCount() > 0) ? true : false);
    }

db.php:

class db {

    public function __construct() {
        $this->dsn = 'mysql:host=127.0.0.1;port=3306;dbname=fb';
        $this->user = 'root';
        $this->password = '';
    }
    public function connect(){
        try {
            $this->conn = new PDO($this->dsn, $this->user, $this->password);
        } catch (PDOException $e) {
            header('Location: /sdfa/signup.php?dbError');
        }
    }

}