How to right unit test code with mockery for Model class

I want to write test case using phpunit for following model function. I'm beginner to unit testing , Kindly please advice me.

public function checkUserEmailExist($email)
{

    $query = \registry::getDBHandler()->prepare("SELECT email FROM users WHERE email = :EMAIL");
    $query->bindValue(":EMAIL", $email);

    try{
        $query->execute();
        $result = ($query->fetchColumn());
    } catch(PDOException $e) {
        \debug::error('MySQL errno ' . $e->getCode() . ': "' . $e->getMessage() . '" when executing: ' . $query->queryString);
    }

    return $result;
}