Quindi una cosa del genere potrebbe andare bene:
Pagina di login:
Codice PHP:
<!DOCTYPE unspecified PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<head>
<?php session_start();?>
<?php require_once 'PhpClass/connection.Class.php'; ?>
<?php require_once 'PhpClass/login.Class.php'; ?>
</head>
<html>
    <body>
    <?php     
            
        $log = new loginClass();
        
        if(!$log->isConnected())
        {
            if(isset($_POST['uname']) && isset($_POST['passw']))
            {        
                $filteredUser = addslashes($_POST['uname']);
                $filteredPass = addslashes($_POST['passw']);
                $connessione = new connect();
                $connessione->connessioneMysql(usert', 'pass', 'host', 'db');    
                $verify = $log->login($filteredUser, $filteredPass);
                if($verify)
                    header("Location: prova.php");
                else
                    header("Location: ".$_SERVER['PHP_SELF']);
            }
            else 
            {                    
    ?>
        Inserire username e passsword
        <form method="post" action="<?php echo $_SERVER['PHP_SELF'];?>">
            Username<input type="text" name="uname">

            Password<input type="password" name="passw">

            <button type="submit">invia</button>
        </form>
    <?php 
            }
        }
        else 
            header("Location: prova.php");
    ?>
    </body>
</html>
e classe per il login:
Codice PHP:
<?php

class loginClass{

    public function isConnected(){
        return (!isset($_SESSION['logged'])) ?  false :  true;
    }
    

    public function login($user, $pass){
        
        $sql="SELECT id FROM users WHERE username=".$user." AND password=".$pass." LIMIT 1";
        $result=mysql_num_rows($sql);
        if($result=='1')
        {
            $_SESSION['logged']='ok';
            return true;
        }
        else     
            return false;
        
    }

}
poi per ogni altra pagina farò il controllo sulla variabile session
Codice PHP:
session_star();
if(isset($_SESSION['logged']))
.....