Access modifiers (public, private, protected) in PHP - Mini Project: Build & Apply

Access Modifiers in PHP Classes

📖 Scenario: You are creating a simple PHP class to represent a bank account. You want to control how the account's balance can be accessed and changed to keep it safe.

🎯 Goal: Build a PHP class called BankAccount with public, private, and protected properties and methods to understand how access modifiers work.

📋 What You'll Learn

Create a class named BankAccount

Add a public property called owner with the owner's name

Add a private property called balance with the initial amount

Add a protected method called formatBalance that returns the balance formatted as a string with a dollar sign

Add a public method called getBalance that uses formatBalance to return the formatted balance

Add a public method called deposit that adds money to the balance

💡 Why This Matters

🌍 Real World

Access modifiers help protect sensitive data in real applications like bank accounts, user profiles, or settings.

💼 Career

Understanding access modifiers is essential for writing secure and maintainable object-oriented code in PHP development jobs.

1

Create the BankAccount class with properties

Create a class called BankAccount with a public property owner set to "Alice" and a private property balance set to 1000.

PHP

# Create the BankAccount class with owner and balance properties
class BankAccount {
    // Your code here
}

Hint

Use public and private keywords before the property names inside the class.

2

Add a protected method to format the balance

Inside the BankAccount class, add a protected method called formatBalance that returns the balance formatted as a string with a dollar sign, like "$1000".

PHP

<?php
class BankAccount {
    public string $owner = "Alice";
    private float $balance = 1000;

    // Add the protected formatBalance method below
    // Your code here
}

Hint

Use protected function formatBalance() and return the balance with a dollar sign.

3

Add public methods to get balance and deposit money

Add two public methods inside BankAccount: getBalance that returns the result of formatBalance(), and deposit that takes a float $amount and adds it to balance.

PHP

balance;
    }

    // Add public getBalance and deposit methods below
    // Your code here
}

Hint

Use public function getBalance() to call formatBalance() and public function deposit(float $amount) to add to balance.

4

Create an object, deposit money, and print the balance

Create a BankAccount object called account. Call deposit(500) on it. Then print the owner's name and the balance using getBalance().

PHP

balance;
    }

    public function getBalance(): string {
        return $this->formatBalance();
    }

    public function deposit(float $amount): void {
        $this->balance += $amount;
    }
}

// Create account, deposit 500, and print owner and balance
// Your code here

Hint

Create the object with new BankAccount(), call deposit(500), then print the owner's name and balance.

Practice

1. Which access modifier allows a property or method to be accessed from anywhere, including outside the class?

easy

A. public

B. private

C. protected

D. static

Solution

1. Step 1: Understand access levels

   Public means accessible from anywhere, private means only inside the class, protected means inside class and subclasses.

2. Step 2: Match the description

   The question asks for access from anywhere, which matches public.

3. Final Answer:

   public -> Option A

4. Quick Check:

   Public = accessible anywhere [OK]

Hint: Public means open to all code everywhere [OK]

Common Mistakes:

• Confusing private with public access
• Thinking protected is accessible outside class
• Mixing static with access modifiers

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

easy

A. private $data;

B. public $data;

C. protected $data;

D. var $data;

Solution

1. Step 1: Recall PHP property declaration syntax

   Access modifiers come before the variable name with a dollar sign, e.g., private $data;

2. Step 2: Identify private declaration

   Only private $data; uses 'private' correctly before the variable.

3. Final Answer:

   private $data; -> Option A

4. Quick Check:

   private property syntax = private $data; [OK]

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

Common Mistakes:

• Using 'var' instead of access modifiers
• Forgetting the dollar sign before variable
• Mixing public or protected when private is needed

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

class Test {
    private $secret = 'hidden';
    public function reveal() {
        return $this->secret;
    }
}
$obj = new Test();
echo $obj->reveal();

medium

A. secret

B. Error: Cannot access private property

C. null

D. hidden

Solution

1. Step 1: Understand private property access inside class

   Private properties can be accessed inside class methods using $this->property.

2. Step 2: Check method reveal() returns private property

   The reveal() method returns $this->secret, which is 'hidden'. So echo prints 'hidden'.

3. Final Answer:

   hidden -> Option D

4. Quick Check:

   Private accessed inside class method = allowed [OK]

Hint: Private properties accessible inside class methods [OK]

Common Mistakes:

• Thinking private properties can't be accessed even inside class
• Expecting error when accessing private inside class
• Confusing property name with string output

4. Identify the error in this PHP code:

class ParentClass {
    protected $value = 10;
}

class ChildClass extends ParentClass {
    public function getValue() {
        return $this->value;
    }
}

$obj = new ParentClass();
echo $obj->value;

medium

A. Syntax error in class declaration

B. ChildClass cannot extend ParentClass

C. Cannot access protected property from outside the class

D. No error, outputs 10

Solution

1. Step 1: Understand protected property access

   Protected properties can be accessed inside the class and child classes, but not from outside.

2. Step 2: Check code usage

   The code tries to echo $obj->value where $obj is ParentClass instance, accessing protected property from outside, causing error.

3. Final Answer:

   Cannot access protected property from outside the class -> Option C

4. Quick Check:

   Protected outside class = error [OK]

Hint: Protected properties not accessible outside class or children [OK]

Common Mistakes:

• Trying to access protected property directly from object
• Confusing protected with public
• Assuming no error when accessing protected externally

5. Given the following PHP classes, what will be the output?

class Base {
    private $data = 'base';
    protected $info = 'protected info';
    public function getData() {
        return $this->data;
    }
}

class Derived extends Base {
    public function getInfo() {
        return $this->info;
    }
    public function getDataFromBase() {
        return $this->data;
    }
}

$obj = new Derived();
echo $obj->getData() . ' & ' . $obj->getInfo() . ' & ' . $obj->getDataFromBase();

hard

A. base & protected info & base

B. base & protected info & Error accessing private property

C. Error accessing private property & protected info & base

D. Error accessing private property & Error accessing protected property & Error

Solution

1. Step 1: Analyze private property access in Derived

   Private property $data is only accessible inside Base class. Derived cannot access $this->data directly.

2. Step 2: Check method calls and outputs

   getData() returns $data from Base (allowed), getInfo() returns protected $info (allowed in Derived), getDataFromBase() tries to access private $data directly (causes error).

3. Final Answer:

   base & protected info & Error accessing private property -> Option B

4. Quick Check:

   Private inaccessible in child = error [OK]

Hint: Private properties not accessible in child classes [OK]

Common Mistakes:

• Assuming private properties are inherited and accessible
• Confusing protected with private access
• Expecting no error when accessing private from child