In PHP, a callback function is a function that is passed as an argument to ______.

In PHP, a callback function is a function that is passed as an argument to another function. It allows the receiving function to call the callback function at a later point in the code. Callback functions are commonly used in PHP for various purposes, such as event handling, dynamic function invocation, and more. The correct option is "Another function" as it covers the general use case of callback functions in PHP. For further information, consult the PHP documentation on callback functions: http://php.net/manual/en/language.types.callable.php