ZEND PHP 5 Certification STUDY GUIDE

120 ” Object Oriented Programming in PHP
point, your destructor will be executed. However, there is no way to determine the
order in which any two objects in your scripts will be destroyed. This can sometimes
cause problems when an object depends on another to perform one or more
functions—for example, if one of your classes encapsulates a database connection
and another class needs that connection to flush its data to the database, you should
not rely on your destructors to perform a transparent flush to the database when
the object is deleted: the instance of the first class that provides database connectivity
could, in fact, be destroyed before the second, thus making it impossible for the
latter to save its data to the database.
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Visibility
PHP 5 adds the notion of object method and property visibility (often referred to as
“PPP”), which enables you to determine the scope from which each component of
your class interfaces can be accessed.
There are four levels of visibility:
public
protected
private
final
The resource can be accessed from any scope.
The resource can only be accessed from within the class
where it is defined and its descendants.
The resource can only be accessed from within the class
where it is defined.
The resource is accessible from any scope, but cannot be
overridden in descendant classes.
The final visibility level only applies to methods and classes. Classes that are declared
as final cannot be extended.
Licensed to 482634 - Amber Barrow (itsadmin@deakin.edu.au)
Typically, you will want to make all API methods and properties public, since you
will want them to be accessible from outside of your objects, while you will want to
keep those used for internal operation as helpers to the API calls protected or private.
Constructors and Destructors—along with all other magic methods (see below)—will
normally be declared as public; there are, however, times when you wish