Properties and visibility in PHP

Introduction

Properties store information inside an object. Visibility controls who can see or change these properties.

When you want to keep some data inside an object private and safe.

When you want to allow other parts of the program to read but not change a value.

When you want to share some information freely with any code.

When you want to organize your code clearly by controlling access to data.

Syntax

PHP

<?php
class ClassName {
    public $property1;    // visible everywhere
    protected $property2; // visible in this class and subclasses
    private $property3;   // visible only inside this class
}
?>

public means anyone can access the property.

protected means only this class and classes that extend it can access the property.

private means only this class can access the property.

Examples

This class has three properties with different visibility levels.

PHP

<?php
class Car {
    public $color;
    protected $engineNumber;
    private $ownerName;
}

You can set public properties from outside the class, but not protected or private ones.

PHP

<?php
$car = new Car();
$car->color = 'red'; // works because color is public
// $car->engineNumber = '123'; // error: protected
// $car->ownerName = 'Alice'; // error: private
?>

A subclass can access protected properties of its parent.

PHP

<?php
class Bike extends Car {
    public function showEngine() {
        return $this->engineNumber; // allowed because protected
    }
}

Sample Program

This program shows how public, protected, and private properties behave. We access protected and private properties using public methods.

PHP

<?php
class Person {
    public $name;
    protected $age;
    private $salary;

    public function __construct($name, $age, $salary) {
        $this->name = $name;
        $this->age = $age;
        $this->salary = $salary;
    }

    public function getAge() {
        return $this->age;
    }

    public function getSalary() {
        return $this->salary;
    }
}

$person = new Person('John', 30, 50000);
echo $person->name . "\n";        // public, accessible
// echo $person->age;              // error: protected
// echo $person->salary;           // error: private

echo $person->getAge() . "\n";    // access protected via method

echo $person->getSalary() . "\n"; // access private via method
?>

Important Notes

Use public for properties you want everyone to access.

Use protected to hide properties but allow child classes to use them.

Use private to keep properties hidden inside the class only.

Summary

Properties hold data inside objects.

Visibility controls who can see or change properties: public, protected, private.

Use methods to access protected or private properties safely.

Practice

1. Which visibility keyword allows a property to be accessed from anywhere, including outside the class?

easy

A. private

B. public

C. protected

D. static

Solution

1. Step 1: Understand visibility keywords

   Public properties can be accessed from anywhere, protected only within the class and subclasses, private only inside the class.

2. Step 2: Identify which keyword allows outside access

   Only public allows access from outside the class.

3. Final Answer:

   public -> Option B

4. Quick Check:

   public = accessible everywhere [OK]

Hint: Public means accessible everywhere, no restrictions [OK]

Common Mistakes:

• Confusing protected with public
• Thinking private allows outside access
• Mixing static with visibility

2. Which of the following is the correct way to declare a private property named $name in a PHP class?

easy

A. private $name;

B. var $name;

C. public $name;

D. protected $name;

Solution

1. Step 1: Recall PHP property declaration syntax

   Properties are declared with visibility keyword followed by variable name and semicolon.

2. Step 2: Identify private property syntax

   Correct syntax is private $name;. The var keyword is outdated and defaults to public.

3. Final Answer:

   private $name; -> Option A

4. Quick Check:

   private property declaration uses 'private' keyword [OK]

Hint: Use 'private' keyword before $property for private visibility [OK]

Common Mistakes:

• Using 'var' instead of visibility keywords
• Omitting the $ sign
• Using public or protected instead of private

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

class Car {
  private $color = 'red';
  public function getColor() {
    return $this->color;
  }
}
$car = new Car();
echo $car->getColor();

medium

A. Empty string

B. Error: Cannot access private property

C. null

D. red

Solution

1. Step 1: Understand private property access

   The property $color is private, so it cannot be accessed directly outside the class.

2. Step 2: Check method usage to access private property

   The public method getColor() returns the private property value, so calling it outputs 'red'.

3. Final Answer:

   red -> Option D

4. Quick Check:

   Private property accessed via public method = 'red' [OK]

Hint: Use public methods to access private properties safely [OK]

Common Mistakes:

• Trying to access private property directly
• Expecting an error when using a getter method
• Confusing null with private property value

4. Identify the error in this PHP class code:

class Person {
  protected $age;
  public function setAge($age) {
    $this->age = $age;
  }
}
$person = new Person();
echo $person->age;

medium

A. Method setAge should be private

B. Missing semicolon after property declaration

C. Cannot access protected property directly outside class

D. Class name should be lowercase

Solution

1. Step 1: Understand protected property visibility

   Protected properties cannot be accessed directly from outside the class or its subclasses.

2. Step 2: Analyze code usage of protected property

   The code tries to echo $person->age directly, which causes an error due to protected visibility.

3. Final Answer:

   Cannot access protected property directly outside class -> Option C

4. Quick Check:

   Protected = no direct outside access [OK]

Hint: Protected properties need methods for outside access [OK]

Common Mistakes:

• Trying to access protected property directly
• Confusing protected with public
• Ignoring visibility rules

5. Given this PHP class, what is the best way to safely update the private property $score only if the new score is higher?

class Game {
  private $score = 0;
  public function updateScore(int $newScore) {
    // What code should go here?
  }
  public function getScore() {
    return $this->score;
  }
}

hard

A. if ($newScore > $this->score) { $this->score = $newScore; }

B. $this->score = $newScore;

C. return $newScore > $this->score;

D. if ($newScore < $this->score) { $this->score = $newScore; }

Solution

1. Step 1: Understand the requirement to update only if new score is higher

   The property $score should change only when $newScore is greater than current $score.

2. Step 2: Choose the correct conditional update code

   if ($newScore > $this->score) { $this->score = $newScore; } checks if $newScore is greater, then updates $score. Other options either overwrite unconditionally or do wrong comparison.

3. Final Answer:

   if ($newScore > $this->score) { $this->score = $newScore; } -> Option A

4. Quick Check:

   Update only if newScore > current score [OK]

Hint: Use if condition to update private property safely [OK]

Common Mistakes:

• Updating score without condition
• Using wrong comparison operator
• Returning boolean instead of updating property