Using {$variable} inside of double quoted strings, is for two reasons:
a) to prevent mistakes like:
$variable = "prefix";
header("Location:somepage.php?value=$variable_name"); // PHP would actually look for a variable called $variable_name instead of the desired $variable
b) allowing to insert variables of arrays and objects into the string without interrupting it
header("Location:somepage.php?value=$myObject->someValue"); // this wouldn't work
header("Location:somepage.php?value={$myObject->someValue}"); // this works
But all of this shouldn't have any effect with Smarty, because Smarty only parses { } entities inside of the template files and header("Location: ...."); definitly doesn't belong there, unless you have a {php}header("Location:...");{/php}
If you have the later one, than you have to access the $event_id differently, because it's not accessible from inside the Smarty class, unless you assign it first with
$smarty->assign('event_id', $event_id);
than either
{php}
$event_id = $this->get_template_vars('event_id');
header("Location:user_event.php?event_id={$event_id}&justadded=1");
{/php}
or
{php}
global $event_id;
header("Location:user_event.php?event_id={$event_id}&justadded=1");
{/php}
But having this kind of code inside the template is quite wrong. Usually such stuff should be done in the actuall php file before the template is ever called.