Xavier's example is rather complicated, and his task would be much more simply accomplished by using classes. Define a base class to do the basic functions open, dump, and close, and create extension classes to override whatever behavior.
class foo {
function open() {
// Default functionality for open()
}
function dump() {
// Default functionality for dump()
}
function close() {
// Default functionality for close()
}
}
class bar extends foo {
function open() {
// Override functionality of open()
}
// dump() and close() remain as in class foo
}
