Pass in the template modifier name, followed by the PHP function that it implements it.
The php-function callback impl 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 a classname and $method being a class method of that class.
See also unregister_modifier(), register_function(), modifiers, Extending Smarty with plugins and Creating Plugin modifiers,