In what way are the users logged in? sessions or cookies? either way this might help:
[PHP]//the blank space is where it changes depending on sessions or cookies.
if( _______ != ''){
$logged_in = 'true';
}else{
$logged_in = 'false';
}
//in the body where ever the download link is
<?php
if($logged_in){ echo '<a href="http://mysite.com/donwload/file.zip">Download</a>';}
?>[/PHP]
TRUE, FALSE or NULL should never be writtin within qoutes - unless you want the TRUE, FALSE or NULL to breated treated as text.
Doing what you have done
[php]
$logged_in = 'true'
//wont check for boolean TRUE or FALSE
//but will assign the text 'true' to that variable
$logged_in = true;
//however, will give the variable a booleab TRUE.
[/php]