No matter your practical experience and understanding of programming and web development, there is one thing no developer can deny; security is one of the major issues that constantly bug the mind throughout the development phase.
Expression - is the cornerstone of PHP. Almost everything you write in PHP is an expression. Expressions are the "bricks" that make up any PHP-programs.
As in any other programming language, PHP has such things as variables.
A constant is a named value that doesn't change while the program (script) in running.
Sometimes you need to initialize an object - that is, to assign initial values to its properties.
PHP does not directly support destructors. However, you can easily simulate a destructor by calling the PHP function unset.
Quite often, when creating an object it is required to set values to some properties. Fortunately, the developers of OOP took this into account and implemented it in the concept of constructor.
Now let's learn how to access to the class members. In PHP there's a special operator for these purposes.