php login system

                                    <?php
session_start();
if(!isset($_POST['pass'])){
    header("Location: index.html");
    exit();
}

$login = $_POST['login'];
$pass = $_POST['pass'];
$login = htmlentities($login, ENT_HTML5, "UTF-8");
$pass = htmlentities($pass, ENT_HTML5, "UTF-8");
require_once "../../includes/connect.php";
try{
    $db = new mysqli($host, $db_user,$db_pass, $db_name);
    if(!$db->connect_errno == 0){
        throw new Exception("connection error");
    }else{
        $query = "SELECT * FROM users WHERE user = ?";
        if(!$exec = $db->prepare($query)){
            throw new mysqli_sql_exception("Query prepare error");
        }else{
            $exec->bind_param("s", $login);
            $exec->execute();
            $res = $exec->get_result();
            $assoc = $res->fetch_assoc();
            if($res->num_rows != 0){
                if(!password_verify($pass,$assoc['pass'])){
                    $_SESSION['error'] = "incorrect login or pass";
                    header("Location: ../../index.html");
                }else{
                    $_SESSION['name'] = $assoc['name'];
                    $_SESSION['surname'] = $assoc['surname'];
                    $_SESSION['desription'] = $assoc['opis'];
                    $_SESSION['role'] = $assoc['role'];
                    if($assoc['isAdmin']){
                        $_SESSION['admin'] = true;
                        header("Location: ../../AdminPanel.php");
                    }else{
                        $_SESSION['loged'] = true;
                        header("Location: ../../User.php");
                    }
                }
            }else{
                $_SESSION['error'] = "Invalid login or Pass";
                header("Location: ../../index.html");
            }
        }
    }
}catch(Exception $e){
    echo $e;
}catch(mysqli_sql_exception $e){
    echo $e;
}

                                    <?php
session_start();// come sempre prima cosa, aprire la sessione 
include("db_con.php"); // Include il file di connessione al database
$_SESSION["username"]=$_POST["username"]; // con questo associo il parametro username che mi è stato passato dal form alla variabile SESSION username
$_SESSION["password"]=$_POST["password"]; // con questo associo il parametro username che mi è stato passato dal form alla variabile SESSION password
$query = mysql_query("SELECT * FROM users WHERE username='".$_POST["username"]."' AND password ='".$_POST["password"]."'")  //per selezionare nel db l'utente e pw che abbiamo appena scritto nel log
or DIE('query non riuscita'.mysql_error());
// Con il SELECT qua sopra selezione dalla tabella users l utente registrato (se lo è) con i parametri che mi ha passato il form di login, quindi
// Quelli dentro la variabile POST. username e password.
if(mysql_num_rows($query)>0){        //se c'è una persona con quel nome nel db allora loggati
$row = mysql_fetch_assoc($query); // metto i risultati dentro una variabile di nome $row
$_SESSION["logged"] =true;  // Nella variabile SESSION associo TRUE al valore logge
header("location:prova.php"); // e mando per esempio ad una pagina esempio.php// in questo caso rimanderò ad una pagina prova.php
}else{
echo "non ti sei registrato con successo"; // altrimenti esce scritta a video questa stringa di errore
}
?>

                                    <?php
session_start();

// initializing variables
$username = "";
$email    = "";
$errors = array(); 

// connect to the database
$db = mysqli_connect('localhost', 'root', '', 'registration');

// REGISTER USER
if (isset($_POST['reg_user'])) {
  // receive all input values from the form
  $username = mysqli_real_escape_string($db, $_POST['username']);
  $email = mysqli_real_escape_string($db, $_POST['email']);
  $password_1 = mysqli_real_escape_string($db, $_POST['password_1']);
  $password_2 = mysqli_real_escape_string($db, $_POST['password_2']);

  // form validation: ensure that the form is correctly filled ...
  // by adding (array_push()) corresponding error unto $errors array
  if (empty($username)) { array_push($errors, "Username is required"); }
  if (empty($email)) { array_push($errors, "Email is required"); }
  if (empty($password_1)) { array_push($errors, "Password is required"); }
  if ($password_1 != $password_2) {
	array_push($errors, "The two passwords do not match");
  }

  // first check the database to make sure 
  // a user does not already exist with the same username and/or email
  $user_check_query = "SELECT * FROM users WHERE username='$username' OR email='$email' LIMIT 1";
  $result = mysqli_query($db, $user_check_query);
  $user = mysqli_fetch_assoc($result);
  
  if ($user) { // if user exists
    if ($user['username'] === $username) {
      array_push($errors, "Username already exists");
    }

    if ($user['email'] === $email) {
      array_push($errors, "email already exists");
    }
  }

  // Finally, register user if there are no errors in the form
  if (count($errors) == 0) {
  	$password = md5($password_1);//encrypt the password before saving in the database

  	$query = "INSERT INTO users (username, email, password) 
  			  VALUES('$username', '$email', '$password')";
  	mysqli_query($db, $query);
  	$_SESSION['username'] = $username;
  	$_SESSION['success'] = "You are now logged in";
  	header('location: index.php');
  }
}

// ... 

                                    <?php
session_start();
$errorMsg = "";
$validUser = $_SESSION["login"] === true;
if(isset($_POST["sub"])) {
  $validUser = $_POST["username"] == "admin" && $_POST["password"] == "password";
  if(!$validUser) $errorMsg = "Invalid username or password.";
  else $_SESSION["login"] = true;
}
if($validUser) {
   header("Location: /login-success.php"); die();
}
?>
<!DOCTYPE html>
<html>
<head>
  <meta http-equiv="content-type" content="text/html;charset=utf-8" />
  <title>Login</title>
</head>
<body>
  <form name="input" action="" method="post">
    <label for="username">Username:</label><input type="text" value="<?= $_POST["username"] ?>" id="username" name="username" />
    <label for="password">Password:</label><input type="password" value="" id="password" name="password" />
    <div class="error"><?= $errorMsg ?></div>
    <input type="submit" value="Home" name="sub" />
  </form>
</body>
</html>

                                    <!DOCTYPE html>
<html>
<head>
	<title>Login</title>
	<script>
  firebase.initializeApp(firebaseConfig);
  const auth = firebase.auth();
  function signUp(){
    var email = document.getElementById("email");
    var password = document.getElementById("password");
    const promise = auth.createUserWithEmailAndPassword(email.value, password.value);
    promise.catch(e => alert(e.message));
    alert("Signed Up");
  }
  function signIn(){
    var email = document.getElementById("email");
    var password = document.getElementById("password");
    const promise = auth.signInWithEmailAndPassword(email.value, password.value);
    promise.catch(e => alert(e.message));
  }
  function signOut(){
    auth.signOut();
    alert("Signed Out");
  }

auth.onAuthStateChanged(function(user){
    if(user){
      var email = user.email;
      alert("Signed in as " + email);
      //Take user to a different or home page

      //is signed in
    }else{
      alert("No Active User");
      //no user is signed in
    }
  });g
	</script>
<style type="text/css">
	body{
	background-color: #55d6aa;
}
h1{
	background-color: #ff4d4d;
	margin: 10px auto;
	text-align: center;
	color: white;
}
#formContainer{
	background-color: white;
	box-shadow: 0 12px 16px 0 rgba(0,0,0,0.24), 0 17px 50px 0 rgba(0,0,0,0.19);

	width: 25%;
	height: 45;
	margin: 10px auto;
}
#header{
	width: 100%;
	height: 10px;
	background: black;
}
#email{
	width: 70%;
	height: 40px;
	display:block;
	margin: 25px auto;
	border: none;
	outline: none;
	border-bottom: 2px solid black;
}
#password{
	width: 70%;
	height: 40px;
	display: block;
	margin: 10px auto;
	border: none;
	outline: none;
	border-bottom: 2px solid black;
}
#signUp{
	background-color: #ff4d4d;
	color: white;
	border: none;
	font-weight: bold;
	padding: 15px 32px;
	border-radius: 10px;
	text-align: center;
	text-decoration: none;
	display: inline-block;
	font-size: 13px;
	margin-top: 20px;
	margin-left: 50px;
}
#signIn{
	background-color: #32ff7e;
	color: white;
	font-weight: bold;
	border: none;
	padding: 15px 35px;
	border-radius: 10px;
	text-align: center;
	text-decoration: none;
	font-size: 13px
}
#signOut{
	background-color: #FFA500;
	color: white;
	border: none;
	padding: 12px 32px;
	border-radius: 10px;
	text-align: center;
	text-decoration: none;
	display: inline-block;
	font-size: 13px;
	margin-top: 9px;
	margin-left: 74px;
	font-weight: bold;
}
button: hover{
box-shadow: 0 12px 16px 0 rgba(0,0,0,0.24), 0 7px 50px 0 rgba(0,0,0,0,.19);
}
</style>
</head>
<body>
	<h1>Login Here</h1>
	<div id="formContainer">
		<div id="header"> </div>
  <input type="email" placeholder="Email" id="email">
  <input type="password" placeholder="Password" id="password">

 <button onclick="signUp()" id="signUp"> Sign Up </button>
  <button onclick="signIn()" id="signIn"> Sign In </button>
  <button onclick="signOut()" id="signOut"> Sign Out </button>
Continue</a>
</body>
</html>