convert value from routes/uri to string

Controller

PHP Code:

public function verification($activation, $email)
{
    $user = $this->model->user('user_activation', $activation);
} 


Model

PHP Code:

public function user($key, $value)
{
    return $this->db->query("
       SELECT *
         FROM users
         WHERE $key = $value // The problem here
     ")->row();
} 


The error message is directed to $ value, the value of $ activation is taken from routes / url.
I try to use single quotes so that it becomes '$ value'. and the results are correct.
my question is whether using single quotes doesn't matter? or is there a better way?