Startup and shutdown functions can be used for one-time
initialization and deinitialization of your modules. As discussed
earlier in this chapter (see the description of the Zend module
descriptor block), there are module, and request startup
and shutdown events.
The module startup and shutdown functions are called whenever a
module is loaded and needs initialization; the request startup and
shutdown functions are called every time a request is processed
(meaning that a file is being executed).
For dynamic extensions, module and request startup/shutdown events
happen at the same time.
Declaration and implementation of these functions can be done with
macros; see the earlier section "Declaration of the Zend Module
Block" for details.