Tjekke om brugeren er logget ind?

Kode 

if ($_SESSION['signed_in'] == false || $_SESSION["logged_in"] = 0)

{
 
echo "
<form method='post' action='authenticate.php'>
	
<td> <input class='login' type='text' name='name' value'Brugernavn'> 

<input class='login' type='password' name='password' value='Kodeord'> 
	
	
<input type='submit' class='loginacc' value='login' name='login' > 
</form> </td>


";
}

Kode 

session_destroy();
   $_SESSION['signed_in'] = false; 
	   $_SESSION["logged_in"] = 0;