Properties and visibility in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code with private property?

Consider this PHP class with a private property. What will be the output when trying to access the private property directly from outside the class?

PHP

<?php
class Box {
    private $content = 'secret';
}
$box = new Box();
echo $box->content;
?>

A Fatal error: Uncaught Error: Cannot access private property Box::$content

B null

C secret

D Warning: Undefined property: Box::$content

💡 Hint

Private properties cannot be accessed directly outside the class.

In PHP, private properties are only accessible inside the class itself. Trying to access them from outside causes a fatal error.

❓ Predict Output

intermediate

What will this PHP code output when accessing a protected property from a subclass?

Look at this PHP code where a subclass tries to access a protected property from its parent. What is the output?

PHP

<?php
class Animal {
    protected $type = 'mammal';
}
class Dog extends Animal {
    public function getType() {
        return $this->type;
    }
}
$dog = new Dog();
echo $dog->getType();
?>

A Warning: Undefined property: Dog::$type

B Fatal error: Cannot access protected property Animal::$type

C null

D mammal

💡 Hint

Protected properties are accessible in subclasses.

Protected properties can be accessed inside the class and its subclasses, so the method returns 'mammal'.

🔧 Debug

advanced

Why does this PHP code cause an error when accessing a static private property?

This PHP code tries to access a private static property from a subclass. What error occurs and why?

PHP

<?php
class ParentClass {
    private static $count = 0;
    public static function increment() {
        self::$count++;
    }
    public static function getCount() {
        return self::$count;
    }
}
class ChildClass extends ParentClass {
    public static function increment() {
        self::$count++;
    }
}
ChildClass::increment();
echo ChildClass::getCount();
?>

A Fatal error: Uncaught Error: Access to undeclared static property ChildClass::$count

B 1

C 0

D Warning: Undefined property: ChildClass::$count

💡 Hint

Private static properties are not accessible in subclasses.

Private static properties belong only to the class where they are declared. The subclass cannot access ParentClass::$count, causing a fatal error.

📝 Syntax

advanced

Which option correctly declares a public property with a default value in PHP 8.1+?

Choose the correct syntax to declare a public property with a default value in a PHP class using typed properties.

A public $name: string = 'John';

B public string $name = 'John';

C public string $name: 'John';

D public $name = string 'John';

💡 Hint

Typed properties use type before the variable name with an equals sign for default.

In PHP 7.4+, typed properties are declared as 'public string $name = "John";'. Other options have invalid syntax.

🚀 Application

expert

How many properties are accessible from outside the class in this PHP code?

Given this PHP class with different visibility properties, how many properties can be accessed directly from an instance outside the class?

PHP

<?php
class Car {
    public $make = 'Toyota';
    protected $model = 'Corolla';
    private $year = 2020;
}
$car = new Car();
// How many properties can be accessed directly like $car->property ?
?>

A 0

B 2

C 1

D 3

💡 Hint

Only public properties are accessible from outside the class.

Only the public property $make can be accessed directly. Protected and private properties are not accessible outside the class.

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