sure... use the header function before any HTML is actually executed to display a HTTP auth box
if($_GET['something'] == 'somethingelse')
{
if(($_SERVER['PHP_AUTH_USER'] == 'username') && ($_SERVER['PHP_AUTH_PW'] == 'password'))
{
echo 'your in';
}
else
{
header('WWW-Authenticate: basic realm="abcde"');
}
}
//edit
its not the most secure method of password protection.. but it can be fine for basic things.
if your looking for something more substantial you need to create a session based auth system using database etc.
Hope this helps you