Server side login-if statement not pointing to mysql query

I am developing a REST web API to communicate with the server. In the function below, I am getting the user's login credentials and return the responses after validation.

The problem is whenever, I login with the correct credentials, it is showing the message 'Invalid Login credentials'.

I am pretty sure that, the values are not checking the conditions or the conditions (I check the email and password fields are not empty and validate the email by FILTER_VALIDATE_EMAIL are wrong.

private function login(){

    if($this->get_request_method() != "GET"){
        $this->response('',406);
    }

    $email = $this->_request['email'];      
    $password = $this->_request['pwd'];         

    if(!empty($email) && !empty($password)){   
        if(filter_var($email, FILTER_VALIDATE_EMAIL)){

            $sql = mysql_query("SELECT id FROM user_login WHERE user_email = '$email' 
                   AND user_password = '".md5($password)."' LIMIT 1", $this->db);

            if(mysql_num_rows($sql) > 0){

                $result = mysql_fetch_array($sql,MYSQL_ASSOC);

                # If success everythig is good send header as "OK" and user details
                $this->response($this->json($result), 200);

            }
            else {
                $message = array('is_data' => 0, 
                                 "message" => "Invalid Login details");
                $this->response($this->json($message), 400);
            }
        }
    }
    # If invalid inputs "Bad Request" status message and reason
    $message = array('code' => "RES3001", 
                     "message" => "Please check the parameters you sent");
    $this->response($this->json($message), 400);
}

It would be very helpful if someone can help me to overcome this issue.