Pass in the template modifier name, followed by the PHP function that it implements it.
The php-function callback function can be either:
A string containing the function name
An array of the form array(&$object, $method) with &$object being a reference to an object and $method being a string containing the method-name
An array of the form array($class, $method) with $class being the class name and $method being a method of the class.
See also unregister_modifier(), register_function(), modifiers section, extending Smarty with plugins and creating plugin modifiers,