Properties and visibility in PHP - Mini Project: Build & Apply

Properties and visibility

📖 Scenario: You are creating a simple PHP class to represent a Book in a library system. Each book has a title and an author. You want to control how these properties can be accessed and changed from outside the class.

🎯 Goal: Build a PHP class called Book with properties that have different visibility levels. Learn how to set and get these properties safely.

📋 What You'll Learn

Create a class named Book

Add a public property called title

Add a private property called author

Add a public method setAuthor to set the author property

Add a public method getAuthor to get the author property

Create an instance of Book and set the title and author

Print the title and author of the book

💡 Why This Matters

🌍 Real World

Controlling access to data inside objects is important in real-world software to protect sensitive information and keep code organized.

💼 Career

Understanding properties and visibility is a key skill for PHP developers working on web applications, APIs, and backend systems.

1

Create the Book class with a public title property

Create a class called Book with a public property named title and set it to an empty string.

PHP

<?php

// Create class Book with public property title
class Book {
    public string $title = "";
}

Hint

Use public string $title = ""; inside the class to create the property.

2

Add a private author property

Inside the Book class, add a private property called author and set it to an empty string.

PHP

<?php

class Book {
    public string $title = "";
    // Add private property author
    private string $author = "";
}

Hint

Use private string $author = ""; to keep the author hidden from outside.

3

Add public setter and getter methods for author

Add two public methods inside Book: setAuthor(string $name) to set the author property, and getAuthor() to return the author property.

PHP

<?php

class Book {
    public string $title = "";
    private string $author = "";

    // Add setAuthor method
    public function setAuthor(string $name) {
        // Your code here
    }

    // Add getAuthor method
    public function getAuthor(): string {
        // Your code here
    }
}

Hint

Use $this->author = $name; inside setAuthor and return $this->author; inside getAuthor.

4

Create a Book instance, set properties, and print them

Create a new Book object called $myBook. Set its title to "1984" and use setAuthor to set the author to "George Orwell". Then print the title and author in the format: Title: 1984, Author: George Orwell.

PHP

author = $name;
    }

    public function getAuthor(): string {
        return $this->author;
    }
}

// Create Book instance and set properties
$myBook = new Book();
// Your code here

Hint

Use $myBook->title = "1984"; and $myBook->setAuthor("George Orwell");. Then print with print("Title: {$myBook->title}, Author: {$myBook->getAuthor()}");.

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