Parent keyword behavior in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the parent class method inside the child class.

PHP

<?php
class Base {
    public function greet() {
        echo "Hello from Base\n";
    }
}

class Child extends Base {
    public function greet() {
        [1]::greet();
    }
}

$child = new Child();
$child->greet();
?>

Drag options to blanks, or click blank then click option'

Aself

Bparent

Cstatic

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using self instead of parent to call the parent method.

Using this which refers to the current object, not the parent.

In PHP, the parent keyword is used to call a method from the parent class inside a child class.

2fill in blank

medium

Complete the code to override the constructor and call the parent constructor.

PHP

<?php
class Animal {
    public function __construct() {
        echo "Animal created\n";
    }
}

class Dog extends Animal {
    public function __construct() {
        [1]::__construct();
        echo "Dog created\n";
    }
}

$dog = new Dog();
?>

Drag options to blanks, or click blank then click option'

Aself

Bthis

Cstatic

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using self::__construct() which calls the current class constructor, causing recursion.

Forgetting to call the parent constructor.

To call the parent class constructor inside the child constructor, use parent::__construct().

3fill in blank

hard

Fix the error in calling the parent method inside a static method.

PHP

<?php
class ParentClass {
    public static function sayHello() {
        echo "Hello from Parent\n";
    }
}

class ChildClass extends ParentClass {
    public static function sayHello() {
        [1]::sayHello();
        echo "Hello from Child\n";
    }
}

ChildClass::sayHello();
?>

Drag options to blanks, or click blank then click option'

Aself

B$this

Cparent

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using $this inside a static method causing an error.

Using self which calls the current class method, not the parent.

Inside a static method, use parent::method() to call the parent static method.

4fill in blank

hard

Fill both blanks to correctly call the parent method and access a parent property.

PHP

<?php
class Vehicle {
    protected static $type = "Vehicle";
    public function getType() {
        return $this->type;
    }
}

class Car extends Vehicle {
    public function getType() {
        echo [1]::getType();
        echo " is a car.";
    }
    public function showType() {
        echo [2]::$type;
    }
}

$car = new Car();
$car->getType();
$car->showType();
?>

Drag options to blanks, or click blank then click option'

Aparent

Bself

CVehicle

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using self instead of parent to call the parent method.

Using parent to access a non-static property causing an error.

Use parent::getType() to call the parent method and Vehicle::$type to access the protected property statically.

5fill in blank

hard

Fill all three blanks to override a method, call the parent method, and add extra behavior.

PHP

<?php
class Logger {
    public function log($msg) {
        echo "Log: $msg\n";
    }
}

class FileLogger extends Logger {
    public function log($msg) {
        [1]::log($msg);
        echo [2] . " - logged to file" . [3];
    }
}

$logger = new FileLogger();
$logger->log("Error occurred");
?>

Drag options to blanks, or click blank then click option'

Aparent

B$msg

C"\n"

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using self instead of parent to call the parent method.

Forgetting to use the variable $msg in the echo statement.

Missing the newline character causing output to be on the same line.

Use parent::log($msg) to call the parent method, then echo the message variable $msg followed by a newline "\n".

Practice

1. What does the parent keyword do in PHP classes?

easy

A. It renames the parent class.

B. It creates a new instance of the parent class.

C. It deletes the parent class methods from the child class.

D. It allows a child class to access methods or properties from its parent class.

Solution

1. Step 1: Understand the role of parent keyword

   The parent keyword in PHP is used inside a child class to refer to its parent class.

2. Step 2: Identify what parent allows

   It allows the child class to call or extend methods or properties defined in the parent class.

3. Final Answer:

   It allows a child class to access methods or properties from its parent class. -> Option D

4. Quick Check:

   parent accesses parent methods/properties [OK]

Hint: Remember: parent calls parent class features [OK]

Common Mistakes:

• Thinking parent creates new objects
• Confusing parent with $this
• Believing parent deletes methods

2. Which of the following is the correct syntax to call a parent class method named show inside a child class method?

easy

A. parent.show();

B. parent->show();

C. parent::show();

D. parent:show();

Solution

1. Step 1: Recall PHP syntax for calling parent methods

   In PHP, to call a parent class method inside a child, use the scope resolution operator :: with parent.

2. Step 2: Match the correct syntax

   The correct syntax is parent::methodName();, so for method show, it is parent::show();.

3. Final Answer:

   parent::show(); -> Option C

4. Quick Check:

   Use :: with parent to call methods [OK]

Hint: Use double colon (::) to call parent methods [OK]

Common Mistakes:

• Using arrow (->) instead of double colon (::)
• Using dot (.) operator which is for strings
• Using single colon (:) which is invalid syntax

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

class A {
  public function greet() {
    return "Hello from A";
  }
}

class B extends A {
  public function greet() {
    return parent::greet() . " and B";
  }
}

$b = new B();
echo $b->greet();

medium

A. Hello from A

B. Hello from A and B

C. Hello from B

D. Error: Cannot call parent method

Solution

1. Step 1: Understand method overriding and parent call

   Class B overrides greet() but calls parent::greet() which returns "Hello from A".

2. Step 2: Concatenate parent result with " and B"

   The child method returns the parent's string plus " and B", so the full string is "Hello from A and B".

3. Final Answer:

   Hello from A and B -> Option B

4. Quick Check:

   parent::greet() + " and B" = "Hello from A and B" [OK]

Hint: parent::method() returns parent's output, can concatenate [OK]

Common Mistakes:

• Ignoring parent:: call and expecting only child's string
• Thinking it causes an error
• Confusing output with only parent's or only child's string

4. Identify the error in this PHP code:

class ParentClass {
  public function sayHi() {
    echo "Hi from parent";
  }
}

class ChildClass extends ParentClass {
  public function sayHi() {
    parent.sayHi();
    echo " and child";
  }
}

$obj = new ChildClass();
$obj->sayHi();

medium

A. Using dot (.) instead of scope resolution (::) to call parent method

B. Missing parentheses after method name

C. Child class cannot override parent methods

D. Parent class method is private

Solution

1. Step 1: Check how parent method is called

   The code uses parent.sayHi(); which is invalid syntax in PHP.

2. Step 2: Correct syntax uses scope resolution operator

   It should be parent::sayHi(); to call the parent method inside child.

3. Final Answer:

   Using dot (.) instead of scope resolution (::) to call parent method -> Option A

4. Quick Check:

   Use parent::method(), not parent.method() [OK]

Hint: Use ::, not . to call parent methods [OK]

Common Mistakes:

• Using dot (.) operator for method calls
• Forgetting parentheses after method name
• Assuming private methods can be called by child

5. Given these classes, what will be the output?

class Base {
  protected function message() {
    return "Base message";
  }
}

class Derived extends Base {
  public function message() {
    return parent::message() . " extended";
  }
}

class MoreDerived extends Derived {
  public function message() {
    return parent::message() . " and more";
  }
}

$obj = new MoreDerived();
echo $obj->message();

hard

A. Base message extended and more

B. Base message and more

C. Error: Cannot access protected method

D. Base message extended

Solution

1. Step 1: Trace method calls through inheritance

   MoreDerived::message() calls parent::message() which is Derived::message().

2. Step 2: Derived::message() calls Base::message()

   Derived calls parent::message() which is Base::message() returning "Base message" and appends " extended".

3. Step 3: Combine all returned strings

   So Derived returns "Base message extended" and MoreDerived appends " and more" resulting in "Base message extended and more".

4. Final Answer:

   Base message extended and more -> Option A

5. Quick Check:

   Chained parent calls build full string [OK]

Hint: Each parent:: call adds to string stepwise [OK]

Common Mistakes:

• Expecting error due to protected method (allowed in child)
• Ignoring chained parent calls
• Assuming only base message prints