Parent keyword behavior in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of parent:: method call in inheritance

What is the output of the following PHP code?

PHP

<?php
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();
?>

A Hello from A and B

B Hello from B

C Hello from A

D Fatal error: Cannot access parent::greet()

💡 Hint

Remember that parent:: calls the method from the parent class.

The method greet() in class B calls parent::greet(), which runs the greet() method from class A, returning "Hello from A". Then it concatenates " and B". So the output is "Hello from A and B".

❓ Predict Output

intermediate

Using parent:: in constructor

What will be the output of this PHP code?

PHP

<?php
class ParentClass {
    public function __construct() {
        echo "Parent constructor called. ";
    }
}
class ChildClass extends ParentClass {
    public function __construct() {
        parent::__construct();
        echo "Child constructor called.";
    }
}
new ChildClass();
?>

A Child constructor called.

B Parent constructor called. Child constructor called.

C Parent constructor called.

D Fatal error: Cannot call parent constructor

💡 Hint

Check how constructors are called in inheritance.

The ChildClass constructor calls parent::__construct(), which runs the ParentClass constructor printing "Parent constructor called. ". Then it prints "Child constructor called.". So the full output is "Parent constructor called. Child constructor called.".

❓ Predict Output

advanced

Output when parent method is private

What happens when a child class tries to call a private parent method using parent:: ?

PHP

<?php
class Base {
    private function secret() {
        return "Secret from Base";
    }
    public function reveal() {
        return $this->secret();
    }
}
class Derived extends Base {
    public function secret() {
        return "Secret from Derived";
    }
    public function revealSecret() {
        return parent::secret();
    }
}
$d = new Derived();
echo $d->reveal();
echo " - ";
echo $d->secret();
echo " - ";
echo $d->revealSecret();
?>

A Fatal error - Secret from Derived - Secret from Base

B Secret from Base - Secret from Derived - Secret from Base

C Secret from Base - Secret from Derived - Fatal error

D Secret from Base - Fatal error - Secret from Derived

💡 Hint

Private methods are not accessible via parent:: in child classes.

Base::reveal() calls $this->secret(). Since secret() is private in Base and private methods cannot be overridden, it calls Base's private secret(), returning "Secret from Base". Derived's public secret() is a separate method, so $d->secret() returns "Secret from Derived". parent::secret() from Derived cannot access Base's private method, causing a fatal error. Output: "Secret from Base - Secret from Derived - Fatal error".

❓ Predict Output

advanced

parent:: with static methods and late static binding

What is the output of this PHP code involving static methods and parent::?

PHP

<?php
class Alpha {
    public static function who() {
        return "Alpha";
    }
    public static function test() {
        return static::who();
    }
}
class Beta extends Alpha {
    public static function who() {
        return "Beta";
    }
    public static function test() {
        return parent::test();
    }
}
echo Beta::test();
?>

A BetaBeta

B Alpha

C Fatal error: Cannot access parent::test()

D Beta

💡 Hint

Remember that static:: uses late static binding to call the method in the called class.

Beta::test() calls parent::test(), which is Alpha::test(). Alpha::test() calls static::who(), which uses late static binding and calls Beta::who(), returning "Beta". So the output is "Beta".

❓ Predict Output

expert

Complex parent:: calls with traits and multiple inheritance

What is the output of this PHP code involving traits and parent:: calls?

PHP

<?php
trait T {
    public function say() {
        return "Trait says hi";
    }
}
class Base {
    public function say() {
        return "Base says hi";
    }
}
class Child extends Base {
    use T;
    public function say() {
        return parent::say() . " and " . T::say();
    }
}
$c = new Child();
echo $c->say();
?>

A Fatal error: Cannot access trait method T::say() directly

B Base says hi and Trait says hi

C Trait says hi and Base says hi

D Base says hi

💡 Hint

Traits methods cannot be called statically like T::say() inside an instance method.

Inside Child::say(), parent::say() correctly calls Base::say(). However, T::say() is a trait method and cannot be called statically like that. This causes a fatal error. The correct way would be to call $this->say() carefully or rename methods to avoid conflict.

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