This function adds a callback function to the list of functions for the specified hook, where hookname is the name of the hook set option. The argument funcname is a string specifying the name of the user-defined function to call. If the optional argument prepend is specified and non-zero, funcname is added to the head of the hook function list. If not specified or zero, funcname is appended to the list

add_hook will not add funcname if it is already present in the hook list.

Refer to Hook functions introduction for a detailed introduction to ACL hooks.