Static properties and methods in PHP

Introduction

Static properties and methods belong to the class itself, not to any specific object. This means you can use them without creating an object first.

When you want to keep a value shared by all objects of a class, like a counter.

When you need a utility function that doesn't depend on object data.

When you want to store configuration or constants related to the class.

When you want to track how many objects of a class have been created.

When you want to call a method without creating an object first.

Syntax

PHP

<?php
class ClassName {
    public static $propertyName = 'value';

    public static function methodName() {
        // code here
    }
}

// Access static property
ClassName::$propertyName;

// Call static method
ClassName::methodName();
?>

Use static keyword to declare static properties and methods.

Access static members with :: operator, not with ->.

Examples

This example shows a static property $count and a static method increment() that increases the count.

PHP

<?php
class Counter {
    public static $count = 0;

    public static function increment() {
        self::$count++;
    }
}

Counter::increment();
echo Counter::$count; // prints 1
?>

This example shows a static method that calculates the square of a number without needing an object.

PHP

<?php
class MathHelper {
    public static function square($num) {
        return $num * $num;
    }
}

echo MathHelper::square(4); // prints 16
?>

Sample Program

This program counts how many Book objects are created using a static property and method.

PHP

<?php
class Book {
    public static $totalBooks = 0;
    public $title;

    public function __construct($title) {
        $this->title = $title;
        self::$totalBooks++;
    }

    public static function getTotalBooks() {
        return self::$totalBooks;
    }
}

$book1 = new Book("PHP Basics");
$book2 = new Book("Learn PHP");

echo "Total books created: " . Book::getTotalBooks();
?>

Important Notes

Inside the class, use self:: to access static properties or methods.

Static properties keep their value shared across all objects of the class.

You cannot use $this inside static methods because there is no object context.

Summary

Static properties and methods belong to the class, not objects.

Use :: to access static members.

They are useful for shared data or utility functions.

Practice

1. Which statement about static properties in PHP is true?
class Example { public static $count = 0; }

easy

A. Static properties belong to the class itself, not to any object instance.

B. Static properties can only be accessed through object instances.

C. Static properties cannot hold values.

D. Static properties are private by default.

Solution

1. Step 1: Understand static property ownership

   Static properties belong to the class, not to individual objects.

2. Step 2: Check access rules

   Static properties are accessed using the class name, not object instances.

3. Final Answer:

   Static properties belong to the class itself, not to any object instance. -> Option A

4. Quick Check:

   Static property = class-level data [OK]

Hint: Static means class-level, not per object [OK]

Common Mistakes:

• Thinking static properties belong to objects
• Trying to access static properties via objects
• Assuming static properties can't hold values

2. Which of the following is the correct syntax to call a static method show() of class Test?

easy

A. $obj->show();

B. $obj::show();

C. Test->show();

D. Test::show();

Solution

1. Step 1: Recall static method call syntax

   Static methods are called using the class name and double colon (::).

2. Step 2: Check each option

   Test::show(); uses Test::show(); which is correct syntax.

3. Final Answer:

   Test::show(); -> Option D

4. Quick Check:

   Static method call = ClassName::method() [OK]

Hint: Use ClassName::method() to call static methods [OK]

Common Mistakes:

• Using object arrow (->) to call static methods
• Using object variable with :: operator incorrectly
• Using class arrow (->) which is invalid

3. What will be the output of the following PHP code?

class Counter {
    public static $count = 0;
    public static function increment() {
        self::$count++;
    }
}
Counter::increment();
Counter::increment();
echo Counter::$count;

medium

A. 1

B. 2

C. 0

D. Error

Solution

1. Step 1: Understand static property and method behavior

   Static property $count starts at 0 and is shared by the class.

2. Step 2: Trace method calls

   Each call to increment() increases $count by 1. Two calls increase it to 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   2 increments = 2 count [OK]

Hint: Static property keeps value across calls [OK]

Common Mistakes:

• Thinking static resets per call
• Confusing self:: with $this->
• Expecting error due to static usage

4. Identify the error in this PHP code snippet:

class Sample {
    public static $value = 10;
    public function show() {
        echo self->$value;
    }
}
$object = new Sample();
$object->show();

medium

A. Cannot use echo inside class methods.

B. Static property cannot be accessed inside non-static methods.

C. Using self::$value instead of self->$value inside the method.

D. Missing static keyword before method declaration.

Solution

1. Step 1: Check static property access syntax

   Static properties must be accessed with self::$propertyName, not self->$propertyName.

2. Step 2: Verify method and property usage

   The method is non-static but can access static properties using self:: syntax.

3. Final Answer:

   Using self::$value instead of self->$value inside the method. -> Option C

4. Quick Check:

   Static property access = self::$property [OK]

Hint: Use :: for static properties, not -> or $ [OK]

Common Mistakes:

• Using $ before property name with self::
• Trying to access static property with $this->
• Confusing static and non-static method rules

5. You want to count how many objects of class User have been created using a static property. Which code correctly implements this?

class User {
    public static $count = 0;
    public function __construct() {
        // What goes here?
    }
}

hard

A. self::$count++;

B. $this->count++;

C. User->count++;

D. static->count++;

Solution

1. Step 1: Understand static property increment in constructor

   Each time an object is created, the constructor runs and should increase the static count.

2. Step 2: Use correct syntax to increment static property

   Use self::$count++; inside the constructor to increment the static property.

3. Final Answer:

   self::$count++; -> Option A

4. Quick Check:

   Increment static with self::$property++ [OK]

Hint: Use self::$property to access static inside class [OK]

Common Mistakes:

• Using $this->count++ which accesses instance property
• Using arrow (->) with class name or static keyword
• Trying to increment static property without self::