Pass in the compiler function name, followed by the PHP function that 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.
cacheable can be omitted in most cases. See Controlling Cacheability of Plugins' Output on how to use it properly.
See also unregister_compiler_function() and Plugin Compiler Functions.