PHP Static Methods and Static Properties

Summary: in this tutorial, you will learn about PHP static methods and static properties. We will show you the differences between  $this and self, and discuss when we can use static methods.

Introduction to PHP static methods and static properties

Sometimes, it is useful if we can access methods and properties in the context of a class rather than an object. To do this, you can use static keyword.

To add a static method to a class, you use the static keyword as follows:

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public static function static_method(){     // method implementation }

You can put the static keyword before or after the method’s visibility. However, by convention, the visibility is declared first.

To add a static property to a class, you also use the static keyword as the following syntax:

1	private static $static_property;

The static methods and static properties are not linked to any particular object of the class but the class itself.

To call a static method outside the class, you use the :: operator as follows:

1	MyClass::static_method();

To access a public static property outside the class, you also use the :: operator:

1	MyClass::$static_property;

However to access static methods and static properties from within an instance of the class, you use  self  instead of  $this as follows:

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<?php class MyClass{ private static $static_property;   public static function static_method(){ //... }   public function method(){ self::$static_property; self::static_method(); } }

Developers, who a new to PHP object-oriented programming, are sometimes confused between  selfand $this. Let’s make the concepts clear.

self vs. $this

$this	self
Represents an instance of the class or object	Represents a class
Always begin with dollar ($) sign	Never begin with dollar($) sign
Is followed by the -> operator	Is followed by the :: operator
The property name after the -> operator does not take dollar ($) sign, e.g., $this->property.	The property name after the :: operator always take the dollar ($) sign.

PHP static methods and properties example

The following example illustrates how to use static method and property:

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<?php class Counter{ private $count; public static $instance;   public function __construct($count = 0){ $this->count = $count; self::$instance++; }   public function count(){ $this->count++; return $this;   }   public function __toString(){ return (string)$this->count; }   public static function countInstance(){ return self::$instance; } }   $c1 = new Counter(); $c1->count()    ->count(); echo 'Counter 1 value: '. $c1 . '<br/>';   $c2 = new Counter(); $c2->count()    ->count()    ->count();   echo 'Counter 2 value: '. $c2 . '<br/>';   echo 'The number of counter instances:' . Counter::countInstance()     . '<br/>';

How it works.

• We declared a new class named Counter that has two properties:  $count and $instance, where  $instance is a static property. Each time a new counter object is created, we increased the value of the  $instance static property by 1. Notice that we accessed the  $instanceproperty by using the self and the :: operator.
• Besides the regular methods, we declared a static method named  countInstance() that is used to return the number of objects instantiated from the Counter class.
• In the test script section below the class, we created two counter objects and displayed the number of instances by calling the countInstance() static method.

When to define static methods

Before defining a static method, ask yourself a question: “Does it make sense to call the method without instantiating a new object?” . If a logic that can be shared by multiple instances of the class, you can extract the code and put it into a static method.

PHP static methods are often used in utility classes of  PHP frameworks. A utility class is a class that contains only static methods.

In this tutorial, we have introduced you PHP static methods and static properties of a class. We then shown you the differences between $this and self , and discussed when we can use static methods.