<?php
function loginUser($user, $pass) {

if(empty($user) || empty($pass)) {
exit();
}else{
$user = addentities($user);
$pass = addentities($pass);
$sql = "select * from users where user = '$user' and pass = '$pass'";
$rs = mysql_query($sql);

if($rs) {

if(mysql_num_rows($rs) != 0) {
$row=@mysql_fetch_object($rs);
$_SESSION['logged'] = "yes";
$_SESSION['user'] = $user;

refreshPage(0, $_SERVER['PHP_SELF']);
}else{

print "Wrong username and password";
}
}
}
}
?>

Now, how simple is to check if a user is logged in?

<?php

if(checkLogged() == false) {

print "You are unauthorized to see this page, please login!";
}elseif(checkLogged() == true) {

print "Welcome $_SESSION[user]";
}
?>

Please note that into head of the files, at the very begining, before outputing anything else you must add session_start(); function! Check this blog for detailed instructions on how to use sessions.

Good luck!