<?php
/**
* User class
* Database Driver  - PDO
*/
class User
{
    private $db; //your database handler

    public function login(string $username, string $password)
    {
      // check if username exists in database
      $check = $this->db->query("SELECT * FROM users WHERE username = '$username' OR email = '$username')->fetchObject();
      if ($check) {
          // verify password
          if (password_verify($password, $check->password) {
              // generate your sessions here
              header("Location: dashboard/index");
           } else {
              // error handling
           }
      }
    }

}

/**
* Using the class in a view
*/

if (isset($_POST['login']) {
   $user = new User; // initalize the User object
   $username = $_POST['username'];
   $password = $_POST['password'];

   $user->login($username, $password);
 }

 /**
 * Procedural style
 */
 .... other db crendials
 if (isset($_POST['login']) {
   $username = $_POST['username'];
   $password = $_POST['password']

   if (!empty($username) && !empty($password)) {
        $check = $db->query("SELECT * FROM users WHERE username = '$username' OR email = '$username')->fetchObject();
        if ($check) {
            // verify password
            if (password_verify($password, $check->password) {
                // generate your sessions here
                header("Location: dashboard/index");
             } else {
                // error handling
             }
        }
    }