Access modifiers (public, private, protected) in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code with access modifiers?

Consider this PHP class and code snippet. What will be printed?

PHP

<?php
class Test {
    public $a = 1;
    private $b = 2;
    protected $c = 3;

    public function printValues() {
        echo $this->a . ',' . $this->b . ',' . $this->c;
    }
}

$test = new Test();
echo $test->a . ',';
// echo $test->b . ','; // Uncommenting this line causes error
// echo $test->c; // Uncommenting this line causes error
$test->printValues();
?>

A 1,1,2,3

B 1,2,3

C 1,1,2

D 1,1,3

💡 Hint

Remember that private and protected properties cannot be accessed directly outside the class, but public properties can.

The public property $a is accessible directly and prints '1,'. The private $b and protected $c cannot be accessed directly outside the class, so those lines are commented out. The printValues() method inside the class can access all three properties and prints '1,2,3'. So the full output is '1,1,2,3'.

❓ Predict Output

intermediate

What error occurs when accessing a private property from outside the class?

What error will this PHP code produce?

PHP

<?php
class Sample {
    private $secret = 'hidden';
}

$obj = new Sample();
echo $obj->secret;
?>

A No output, prints 'hidden'

B Parse error: syntax error, unexpected 'private'

C Fatal error: Uncaught Error: Cannot access private property Sample::$secret

D Notice: Undefined property: Sample::$secret

💡 Hint

Private properties cannot be accessed from outside the class.

Trying to access a private property directly from outside the class causes a fatal error in PHP. The error message says you cannot access the private property.

🔧 Debug

advanced

Why does this code cause an error when accessing a protected property?

Look at this PHP code. Why does it cause an error?

PHP

<?php
class ParentClass {
    protected $value = 10;
}

class ChildClass extends ParentClass {
}

$obj = new ChildClass();
echo $obj->value;
?>

A Protected properties cannot be accessed directly outside the class or its children

B Protected properties can only be accessed inside the parent class, not children

C The property <code>$value</code> is not declared public, so it is undefined

D PHP does not support protected properties

💡 Hint

Think about where protected properties can be accessed from.

Protected properties can be accessed inside the class where they are declared and in child classes, but not from outside objects. Accessing $obj->value from outside causes an error.

📝 Syntax

advanced

Which option correctly declares a private property and a public method in PHP?

Choose the correct PHP code snippet that declares a private property $data and a public method getData() that returns it.

A class Example { private $data; public getData() { return $this->data; } }

B class Example { private $data; function getData() { return $data; } }

C class Example { public $data; private function getData() { return $this->data; } }

D class Example { private $data; public function getData() { return $this->data; } }

💡 Hint

Remember to use function keyword and $this-> to access properties inside methods.

Option D correctly declares a private property $data and a public method getData() that returns $this->data. Option D misses public and uses $data without $this->. Option D swaps visibility and method access. Option D misses function keyword.

🚀 Application

expert

How many properties are accessible inside this child class method?

Given these classes, how many properties can the showProperties() method access?

PHP

<?php
class Base {
    public $pub = 1;
    private $priv = 2;
    protected $prot = 3;
}

class Derived extends Base {
    public function showProperties() {
        $vars = get_object_vars($this);
        $count = 0;
        if (isset($vars['pub'])) $count++;
        if (isset($vars['priv'])) $count++;
        if (isset($vars['prot'])) $count++;
        return $count;
    }
}

$obj = new Derived();
echo $obj->showProperties();
?>

A 2

B 3

C 1

D 0

💡 Hint

Think about which properties are accessible inside a child class method.

The child class method can access public and protected properties from the parent class, but not private ones. So it counts $pub and $prot, but not $priv. The count is 2.

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