The callback array looks like this:
{{code: php
$callbacks = array(
static method call
array('Class_Name', 'method', $param1, $param2, ...),
instance method call on a registry object
array('registry-key', 'method', $param1, $param2, ...),
instance method call
array($object, 'method', $param1, $param2, ...),
function call
array(null, 'function', $param1, $param2, ...),
file include, as in previous versions of Solar
'path/to/file.php',
);
}}