<?php
include('sql.php');
$username = $_POST['username'];
$password = $_POST['password'];
$confirm = $_POST['confirm'];

if($username=='' || $password=''){header('Location:/register.php');}
if($password!=$confirm){header('Location:/register.php');}
$sql = "INSERT INTO login (username, password) VALUES ('".$username."', '".password_hash($password, PASSWORD_DEFAULT)."')";
if (mysqli_query($mysqli, $sql)) {
  header('Location: /dashboard.php');
} else {
  echo $mysqli->error;
}
mysqli_close($mysqli);

<?php
session_start();
$error='';

if (isset($_POST['submit'])) {
  if (empty($_POST['username']) || empty($_POST['password'])) {
    header("Location: /admin.php?error=invalid");
  } else {
    include('sql.php');
    $username = mysqli_real_escape_string($mysqli,stripslashes($_POST['username']));
    $password = mysqli_real_escape_string($mysqli,stripslashes($_POST['password']));
    $sql="SELECT * FROM login WHERE username='".$username."'";
    $result=$mysqli->query($sql);
    if ($result->num_rows == 1) {
      while($row = $result->fetch_assoc()) {
        $verify = password_verify($password, $row['password']);
        if($verify==false){
          header("Location: /admin.php?error=mismatch");
        } else {
          $_SESSION['login_user']=$username;
          $_SESSION['login_pass']=$password;
          if($_POST['stay']=='stay'){
            setcookie('username', $username, time()+31536000, '/');
            setcookie('password', $password, time()+31536000, '/');
          }
          header("location: /dashboard.php");
        }
      }
    } else {
      header("Location: /admin.php?error=mismatch");
    }
    mysqli_close($mysqli);
  }
}