smarty/docs/programmers/smarty-constants.md

Constants

SMARTY_DIR

This is the full system path to the location of the Smarty class files. If this is not defined in your script, then Smarty will attempt to determine the appropriate value automatically. If defined, the path must end with a trailing slash/.

<?php
// set path to Smarty directory *nix style
define('SMARTY_DIR', '/usr/local/lib/php/Smarty-v.e.r/libs/');

// path to Smarty windows style
define('SMARTY_DIR', 'c:/webroot/libs/Smarty-v.e.r/libs/');

// include the smarty class, note 'S' is upper case
require_once(SMARTY_DIR . 'Smarty.class.php');
?>

See also $smarty.const and $php_handling constants