LOGIN.PHP

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Area Riservata | Autenticati</title>

        <link rel="stylesheet" href="css/style2.css">

  </head>

  <body>

<?php
session_start();

function Open($dbname) {

   $con = new mysqli("127.0.0.1","root","",$dbname);

   if ($con->connect_errno) {
     echo "<h2>".$con->connect_error."</h2>";
     exit();
   }
   else

   return $con;
 }

 function Query($con,$sql) {
    $res = $con->query($sql);
       if (!$res)
      echo "<h2>".$con->error."</h2>";
    return $res;
 }

 function Close($con) {
    $con->close();
 }

if ($_SERVER['REQUEST_METHOD'] == 'POST')
 {

     $username = $_POST['username'];
         $password = $_POST['password'];

         $mysql = Open("catdb");
         $sql = "SELECT ID_Tecnico, Nome, Cognome
                 FROM tecnico
                 WHERE Username= '$username' && Password='$password'";
         $rs = Query($mysql, $sql);
         Close($mysql);
           if ($rs->num_rows == 1) {
                $obj = $rs->fetch_object();
                $_SESSION['id_tecnico'] = $obj->Id_Tecnico;
                $_SESSION['nome'] = $obj->Nome;
                $_SESSION['cognome'] = $obj->Cognome;
                                header("Location: areariservata.php");
     }
     else  {
    echo "<div class='container'>

  <div class='login'>

    <h1 class='login-heading'>

      <strong>Dati errati!</strong> Torna indietro.</h1>

    <button class='btn btn--right' onclick='history.back()'> Indietro </button>

    </div>
</div>";
}

 } else

echo "

<div class='container'>

  <div class='login'>

    <h1 class='login-heading'>

      <strong>Benvenuto.</strong> Autenticati.</h1>

<form method='post' action='" . $_SERVER['PHP_SELF'] ."'>

        <input type='text' name='username' placeholder='Username' required='required' class='input-txt' />

          <input type='password' name='password' placeholder='Password' required='required' class='input-txt' />

            <button type='submit' class='btn btn--right'>Entra</button>

          </div>
      </form>

</div>
</div>";

?>

  </body>
</html>