Defining properties

Learn to define properties inside a class and give default values.

Defining properties

A property is a variable stored inside a class. A property is defined with a visibility keyword (e.g. public) and a variable name starting with $.

Example: public $title = "Unknown";. Here public means the property can be accessed from outside, $title is the variable name and "Unknown" is the default value. The default value is optional; if omitted, the property is null at first.

Default value and visibility

Giving a property a default value provides ready initial data when an instance is created. The default must be a constant; it cannot be a variable or function call.

Besides public there are private and protected visibilities. We will only use public in this lesson; the others come later. A public property can be accessed from outside the class too.

Exercise Add a property to the Book class

Define a class named Book. Add the property public $title = "Unknown"; inside it. After the class, write echo "Book class ready" . "\n";.

Show example solution

Inside class Book { } add public $title = "Unknown";, then echo.
Example solution:
<?php
// Define the Book class and the $title property
class Book {
    public $title = "Unknown";
}
echo "Book class ready" . "\n";

Test yourself

Complete the property definition

class Book {
    ____ $title = "Unknown";
}

Show answer

class Book {
    public $title = "Unknown";
}

Which is a valid property definition inside a class?
- $title = "Unknown";
- property $title = "Unknown";
- public $title = "Unknown";
- var title = "Unknown";
Show answer

public $title = "Unknown"; — Correct. The public visibility, the $title variable and a default value form a property.
Match the parts of a property

public

Visibility
$title

Property variable
"Unknown"

Default value

Next lesson: Defining methods

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