Pass in the template function name, followed by the PHP function name that 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.
cacheable and cache_attrs can be omitted in most cases. See controlling cacheability of plugins output on how to use them properly.
See also unregister_function() and the plugin functions section.