[Opserty]

PHP Code:

$prevweek = time() - 7776000;

if ($userinfo[lastactivity] < $prevweek )
{
    $olduser = 1;
}
else
{
    $olduser = 0;
} 


<if condition="$olduser == 1">
    User has not logged in for 1 week!
</if>

You were only missing a second equals sign in the conditional and its condition not conditional but I made the rest of it look nice. It should work now.

You may want to add a new plugin instead of editing a file, a suitable hook location would be something like: memberinfo_complete.