Plugins can be either loaded by Smarty automatically from the filesystem or they can be registered at runtime via one of the register_* API functions. They can also be unregistered by using unregister_* API functions. For the plugins that are registered at runtime, the name of the plugin function(s) does not have to follow the naming convention. If a plugin depends on some functionality provided by another plugin (as is the case with some plugins bundled with Smarty), then the proper way to load the needed plugin is this: require_once SMARTY_DIR . 'plugins/function.html_options.php'; As a general rule, Smarty object is always passed to the plugins as the last parameter (except for modifiers).