Insert class in PHP does not work and shows no error

I have the following code in php :

<?php

class Users extends DB {

    private function verifyUser($email) {
        $select = self::conn()->prepare("SELECT * FROM 'users' WHERE email = '{$email}'");
        $select->execute();

        if ($select->rowCount() >= 1) {
            return true;
        } else {
            return false;
        }
    }

    public function insertUser($data = array()) {
        if ($this->verifyUsers($data[2])) {
            return false;
        } else {
            $insert = "INSERT INTO 'users' (email) VALUES (?,?,?)";
            $stmt = self::conn()->prepare($insert);

            if ($stmt->execute($data)) {
                return true;
            } else {
                return false;
            }
        }
    }
}

It is not working, nor does it show errors . Can anyone help?