How to make a website restricted only for users that are logged in? [closed]

So i made a website to test a login system for my project and after some work with myysql and php i made a working login system. The only problem is that I don’t know how to restrict a website only for users that are logged in. Below is the code of the login page itself and the authentication.php.

index.php (login):

<html>  
<head>  
    <title>PHP login system</title>  
    <link rel = "stylesheet" type = "text/css" href = "style.css">   
</head>  
<body>
    <div id = "frm">  
        <h1>Login</h1>  
        <form name="f1" action = "authentication.php" onsubmit = "return validation()" method = "POST">  
            <p>  
                <label> UserName: </label>  
                <input type = "text" id ="user" name  = "user" />  
            </p>  
            <p>  
                <label> Password: </label>  
                <input type = "password" id ="pass" name  = "pass" />  
            </p>  
            <p>     
                <input type =  "submit" id = "btn" value = "Login" />  
            </p>  
        </form>  
    </div>   
    <script>  
            function validation()  
            {  
                var id=document.f1.user.value;  
                var ps=document.f1.pass.value;  
                if(id.length=="" && ps.length=="") {  
                    alert("User Name and Password fields are empty");  
                    return false;  
                }  
                else  
                {  
                    if(id.length=="") {  
                        alert("User Name is empty");  
                        return false;  
                    }   
                    if (ps.length=="") {  
                    alert("Password field is empty");  
                    return false;  
                    }  
                }                             
            }  
        </script>  
</body>     
</html>  

authentication.php:

<?php     
    include('connection.php');  
    $username = $_POST['user'];  
    $password = $_POST['pass'];  
      
        //to prevent from mysqli injection  
        $username = stripcslashes($username);  
        $password = stripcslashes($password);  
        $username = mysqli_real_escape_string($con, $username);  
        $password = mysqli_real_escape_string($con, $password);  
      
        $sql = "select *from login where username = '$username' and password = '$password'";  
        $result = mysqli_query($con, $sql);  
        $row = mysqli_fetch_array($result, MYSQLI_ASSOC);  
        $count = mysqli_num_rows($result);  
          
        if($count == 1){  
            header("location:site.php");
        }  
        else{
            $user='0';  
            header("location:index.php");
        }     
?>   

site.php (the site you should se after logging in)

<?php
echo"example website";
?>

So I want authentication.php to redirect me to site.php and prevention from user accesing site.php directly without logging in (the user should login everytime he wants to access the site.