Instance vs Static Properties

Part of the Object Oriented Programming section of Coddy's PHP journey — lesson 15 of 91.

When you create a property in a class, you need to decide whether it belongs to each individual object or to the class itself. This distinction is the difference between instance properties and static properties.

Instance properties are the default. Each object gets its own copy of the property, and changing it in one object doesn't affect others:

<?php
class User {
    public $name;
    
    public function __construct($name) {
        $this->name = $name;
    }
}

$alice = new User("Alice");
$bob = new User("Bob");

echo $alice->name . "\n";
echo $bob->name . "\n";

Output:

Alice
Bob

Static properties belong to the class, not to any specific object. You declare them with the static keyword and access them using the class name with :: instead of an object with ->:

<?php
class User {
    public static $count = 0;
    public $name;
    
    public function __construct($name) {
        $this->name = $name;
        self::$count++;
    }
}

$alice = new User("Alice");
$bob = new User("Bob");

echo User::$count;

Output:

Notice how $count is shared across all instances. Every time we create a new User, the same $count variable increments. Inside the class, we use self::$count to reference the static property.

Key Point: Use instance properties when each object needs its own value. Use static properties when you need to share data across all objects of a class, like counting instances or storing configuration.

Challenge

Easy

Let's build a visitor tracking system that demonstrates the difference between instance properties (unique to each visitor) and static properties (shared across all visitors).

You'll create two files to organize your code:

Visitor.php — Define a Visitor class that tracks individual visitors and keeps a running count of total visitors. Each visitor should have their own $name property (instance property), while a $totalVisitors property should be shared across all instances (static property). The constructor accepts a name, sets it, and increments the total visitor count. Add a getInfo() method that returns "[name] is visitor #[totalVisitors]".
main.php — Include the Visitor class file, then create three visitors with the names "Alice", "Bob", and "Charlie" (in that order). After creating all three, print the result of calling getInfo() on each visitor (each on its own line), then print the total visitor count by accessing the static property directly from the class using Visitor::$totalVisitors.

Remember that inside the class, you access static properties using self::$propertyName, while from outside the class you use ClassName::$propertyName. Instance properties are accessed with $this->propertyName as usual.

Your output should show each visitor with their assigned number, followed by the final total count on the last line.

Cheat sheet

Instance properties belong to each individual object, while static properties belong to the class itself and are shared across all instances.

Instance Properties:

Each object gets its own copy of the property:

<?php
class User {
    public $name;
    
    public function __construct($name) {
        $this->name = $name;
    }
}

$alice = new User("Alice");
$bob = new User("Bob");

echo $alice->name; // Alice
echo $bob->name;   // Bob

Static Properties:

Declared with the static keyword and shared across all instances:

<?php
class User {
    public static $count = 0;
    public $name;
    
    public function __construct($name) {
        $this->name = $name;
        self::$count++;
    }
}

$alice = new User("Alice");
$bob = new User("Bob");

echo User::$count; // 2

Accessing Static Properties:

Inside the class: self::$propertyName
Outside the class: ClassName::$propertyName

Accessing Instance Properties:

Inside the class: $this->propertyName
Outside the class: $object->propertyName

Try it yourself

<?php
// Include the Visitor class
require_once 'Visitor.php';

// TODO: Create three visitors with names "Alice", "Bob", and "Charlie" (in that order)

// TODO: Print the result of getInfo() for each visitor (each on its own line)

// TODO: Print the total visitor count using Visitor::$totalVisitors
?>

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Instance vs Static Properties

Cheat sheet

All lessons in Object Oriented Programming