I'm trying to build my first object oriented system. I'm not sure that my register function below is the best way of approaching this within OOP

class User {
  public $username;
  private $email;
  private $password;

  public function register() {
   //Check if the username input is set.
   if(isset( $_POST['username'] )) {
     //Assign the variables.
     $this->username = $_POST['username'];
     $this->email = $_POST['email'];
     $unhashed_pass = $_POST['password'];
  
     //Hash the password   
     $this->password = password_hash($unhashed_pass, PASSWORD_BCRYPT);

     $sql = "INSERT INTO 'users' ('id', 'username', 'email', 'password') VALUES (NULL, '$this->username', '$this->email', '$this->password')";
     $results = mysql_query($sql);

     if($results) {
      //Query was successful
      echo "Success";
     } else {
      echo mysql_error();
     }
   }
  }
}

Please give me any feedback. I'm okay with PHP just trying to learn how to effectively use OOP.

I'm trying to build my first object oriented system. I'm not sure that my register function below is the best way of approaching this within OOP.

class User {
  public $username;
  private $email;
  private $password;

  public function register() {
   //Check if the username input is set.
   if(isset( $_POST['username'] )) {
     //Assign the variables.
     $this->username = $_POST['username'];
     $this->email = $_POST['email'];
     $unhashed_pass = $_POST['password'];
  
     //Hash the password   
     $this->password = password_hash($unhashed_pass, PASSWORD_BCRYPT);

     $sql = "INSERT INTO 'users' ('id', 'username', 'email', 'password') VALUES (NULL, '$this->username', '$this->email', '$this->password')";
     $results = mysql_query($sql);

     if($results) {
      //Query was successful
      echo "Success";
     } else {
      echo mysql_error();
     }
   }
  }
}

Please give me any feedback. I'm okay with PHP; I'm just trying to learn how to effectively use OOP.