how in workspaces can I connect to a a database?

<?php
namespace Tutor\Provider;

use Silex\Application;
use Silex\ServiceProviderInterface;
use Symfony\Component\HttpFoundation\Session\Session;

class Tutor {

    private $link;

    public function __construct() {
        $this->$link = new mysqli_connect("localhost","mayur","","londontutors"); //here what should I change this to?
    }

    public function get_tutor($username) {
        $username = mysqli_real_escape_string($this->link, $username);
        $result = mysqli_query($this->link, "select tutor.email, name, username, subjects, summary, about, image from tutor inner join user on tutor.email = user.email where username = '$username'");
        if ($tutor = mysqli_fetch_assoc($result)) {
            $result = mysqli_query($this->link, "select subject from tutorexperience where email = '{$tutor['email']}'");
            $tutor['experience'] = array();
            while ($row = mysqli_fetch_assoc($result)) {
                $tutor['experience'][] = $row['subject'];
            }
            $result = mysqli_query($this->link, "select qualification from tutorqualification where email = '{$tutor['email']}'");
            $tutor['qualifications'] = array();
            while ($row = mysqli_fetch_assoc($result)) {
                $tutor['qualifications'][] = $row['qualification'];
            }
            return $tutor;
        } else {
            return null;
        }
    }

    public function get_tutors() {
        $result = mysqli_query($this->link, "select tutor.email, name, username, subjects, summary, about, image from tutor inner join user on tutor.email = user.email");
        $tutors = array();
        while ($tutor = mysqli_fetch_assoc($result)) {
            $tutors[] = $tutor;
        }
        return $tutors;
    }

    public function get_group_tuition($email) {
        $email = $email || '';
        $email = mysqli_real_escape_string($this->link, $email);
        $result = mysqli_query($this->link, "select tutoremail, starttime, endtime, location, class, level, topic, capacity, (capacity - (select count(*) from grouptuitionbooking where grouptuitionbooking.tutoremail = grouptuition.tutoremail and grouptuitionbooking.starttime = grouptuition.starttime)) as available, username, name, (select '$email' in (select studentemail from grouptuitionbooking where grouptuitionbooking.tutoremail = grouptuition.tutoremail and grouptuitionbooking.starttime = grouptuition.starttime)) as booked from grouptuition inner join (select tutor.email, tutor.username, user.name from tutor inner join user on tutor.email = user.email) t on grouptuition.tutoremail = t.email and starttime >= NOW() order by starttime asc, username asc");
        $groups = array();
        while ($group = mysqli_fetch_assoc($result)) {
            $group['datetime'] = date("l jS g", strtotime($group['starttime'])) . '-' . date("ga", strtotime($group['endtime']));
            $groups[] = $group;
        }
        return $groups;
    }

    public function get_group_tuition_details($tutoremail, $starttime) {
        $tutoremail = mysqli_real_escape_string($this->link, $tutoremail);
        $starttime = mysqli_real_escape_string($this->link, $starttime);
        $result = mysqli_query($this->link, "select tutoremail, starttime, endtime, location, class, level, topic, capacity, (capacity - (select count(*) from grouptuitionbooking where grouptuitionbooking.tutoremail = grouptuition.tutoremail and grouptuitionbooking.starttime = grouptuition.starttime)) as available, username, name from grouptuition inner join (select tutor.email, tutor.username, user.name from tutor inner join user on tutor.email = user.email) t on grouptuition.tutoremail = t.email and grouptuition.tutoremail = '{$tutoremail}' and grouptuition.starttime = '{$starttime}' order by starttime asc, username asc");
        if ($group = mysqli_fetch_assoc($result)) {
            $group['datetime'] = date("l jS g", strtotime($group['starttime'])) . '-' . date("ga", strtotime($group['endtime']));
            $result = mysqli_query($this->link, "select email, name, address, phone from grouptuitionbooking inner join user on studentemail = email and tutoremail = '{$tutoremail}' and starttime = '{$starttime}'");
            $group['students'] = array();
            while ($student = mysqli_fetch_assoc($result)) {
                $group['students'][] = $student;
            }
            return $group;
        } else {
            return null;
        }
    }
}

class TutorServiceProvider implements ServiceProviderInterface {

    public function register(Application $app) {
        $app['tutor'] = function () {
            return new Tutor();
        };
    }

    public function boot(Application $app) {
    }
}
?>