Constructor inheritance in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a constructor in the parent class.

PHP

<?php
class ParentClass {
    public function [1]() {
        echo "Parent constructor called\n";
    }
}
?>

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

AParentClass

Bconstruct

C__construct

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name without underscores like 'construct'.

Using the class name as constructor (old PHP style).

The constructor method in PHP is named __construct. This special method is called automatically when an object is created.

2fill in blank

medium

Complete the code to call the parent constructor from the child class.

PHP

<?php
class ChildClass extends ParentClass {
    public function __construct() {
        [1]();
        echo "Child constructor called\n";
    }
}
?>

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

Aparent::ParentClass

Bthis->__construct

Cself::__construct

Dparent::__construct

Attempts:

3 left

💡 Hint

Common Mistakes

Calling self::__construct() which calls the current class constructor recursively.

Using this instead of parent.

To call the parent class constructor in PHP, use parent::__construct().

3fill in blank

hard

Fix the error in the child constructor to properly inherit the parent constructor.

PHP

<?php
class ChildClass extends ParentClass {
    public function __construct() {
        [1];
        echo "Child constructor called\n";
    }
}
?>

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

Aparent::__construct()

Bparent.__construct()

Cparent->__construct()

Dparent::__construct

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses when calling the method.

Using arrow -> or dot . instead of double colon.

The correct syntax to call the parent constructor is parent::__construct() with parentheses to invoke the method.

4fill in blank

hard

Fill both blanks to create a child constructor that calls the parent constructor and sets a property.

PHP

<?php
class ChildClass extends ParentClass {
    public $name;
    public function __construct($name) {
        [1]();
        $this->[2] = $name;
    }
}
?>

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

Aparent::__construct

Bname

Cparent::__construct()

Dname()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling parent constructor without parentheses.

Using incorrect property name or syntax.

The child constructor calls the parent constructor with parent::__construct() and sets the property $name using $this->name = $name;.

5fill in blank

hard

Fill all three blanks to complete the child constructor that calls the parent constructor with a parameter and sets a property.

PHP

<?php
class ParentClass {
    public function __construct($id) {
        echo "Parent constructor with ID: $id\n";
    }
}

class ChildClass extends ParentClass {
    public $name;
    public function __construct($id, $name) {
        [1]([2]);
        $this->[3] = $name;
    }
}
?>

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

Aparent::__construct

B$id

Cname

Dparent::__construct()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling parent constructor without passing the parameter.

Using incorrect syntax for calling the parent constructor.

The child constructor calls the parent constructor passing $id using parent::__construct($id); and sets the $name property.

Practice

1. What happens if a child class in PHP does not define its own constructor but the parent class has one?

easy

A. PHP throws an error because the child lacks a constructor.

B. No constructor is called at all.

C. The parent's constructor is automatically called when creating a child object.

D. The child class inherits the parent's constructor as its own.

Solution

1. Step 1: Understand default constructor behavior

   If the child class does not define a constructor, PHP automatically calls the parent's constructor when creating a child object.

2. Step 2: Confirm no error or override

   Since the child has no constructor, no error occurs and the parent's constructor runs as expected.

3. Final Answer:

   The parent's constructor is automatically called when creating a child object. -> Option C

4. Quick Check:

   Child without constructor calls parent constructor [OK]

Hint: No child constructor? Parent constructor runs automatically [OK]

Common Mistakes:

• Thinking child constructor must exist to call parent constructor
• Assuming PHP throws error if child lacks constructor
• Believing child inherits parent's constructor as a method

2. Which of the following is the correct syntax to call a parent class constructor inside a child class constructor in PHP?

easy

A. parent::__construct();

B. parent::constructor();

C. super::__construct();

D. parent->__construct();

Solution

1. Step 1: Recall PHP parent constructor call syntax

   In PHP, the parent's constructor is called using parent::__construct(); inside the child's constructor.

2. Step 2: Eliminate incorrect syntax

   Options A, C, and D use invalid syntax: 'constructor' is not the method name, 'super' is not PHP keyword, and 'parent->' is incorrect.

3. Final Answer:

   parent::__construct(); -> Option A

4. Quick Check:

   Use parent::__construct() to call parent constructor [OK]

Hint: Use parent::__construct() exactly to call parent constructor [OK]

Common Mistakes:

• Using 'super' keyword instead of 'parent'
• Calling parent constructor as parent::constructor()
• Using object operator '->' with parent keyword

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

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

medium

A. Child constructor called.

B. Parent constructor called. Child constructor called.

C. Parent constructor called.

D. No output.

Solution

1. Step 1: Trace constructor calls

   The child class constructor calls parent::__construct(); first, so the parent's constructor runs and prints "Parent constructor called. "

2. Step 2: Continue child constructor execution

   After the parent's constructor, the child constructor prints "Child constructor called." immediately after.

3. Final Answer:

   Parent constructor called. Child constructor called. -> Option B

4. Quick Check:

   Child calls parent constructor then prints own message [OK]

Hint: parent::__construct() runs parent's constructor first [OK]

Common Mistakes:

• Ignoring parent::__construct() call in child constructor
• Assuming only child constructor output appears
• Thinking parent's constructor is not called explicitly

4. Identify the error in the following PHP code related to constructor inheritance:

class Base {
  public function __construct() {
    echo "Base constructor.";
  }
}
class Derived extends Base {
  public function __construct() {
    parent::construct();
    echo "Derived constructor.";
  }
}
new Derived();

medium

A. The parent constructor is called with wrong method name 'construct()'.

B. The child class should not have a constructor.

C. Missing call to parent's constructor causes error.

D. No error; code runs fine.

Solution

1. Step 1: Check parent constructor call syntax

   The code calls parent::construct(); but the correct method name is __construct() with double underscores.

2. Step 2: Understand impact of wrong method name

   Because 'construct()' does not exist, PHP will throw a fatal error or warning about undefined method.

3. Final Answer:

   The parent constructor is called with wrong method name 'construct()'. -> Option A

4. Quick Check:

   Use parent::__construct(), not parent::construct() [OK]

Hint: Always use double underscore __construct() for constructors [OK]

Common Mistakes:

• Calling parent::construct() instead of parent::__construct()
• Forgetting double underscores in constructor name
• Assuming parent constructor runs without correct call

5. Given the following classes, what will be the output when creating a new Child object?

class Parent {
  public function __construct() {
    echo "Parent start. ";
  }
}
class Child extends Parent {
  public function __construct() {
    echo "Child start. ";
    parent::__construct();
    echo "Child end.";
  }
}
new Child();

hard

A. Parent start. Child end.

B. Parent start. Child start. Child end.

C. Child start. Child end.

D. Child start. Parent start. Child end.

Solution

1. Step 1: Follow the order of echo statements

   The child constructor first prints "Child start. ". Then it calls parent::__construct(); which prints "Parent start. ". Finally, the child prints "Child end.".

2. Step 2: Combine outputs in sequence

   Putting all together, the output is "Child start. Parent start. Child end." exactly in that order.

3. Final Answer:

   Child start. Parent start. Child end. -> Option D

4. Quick Check:

   Child prints before and after parent constructor call [OK]

Hint: Order matters: child's code runs before and after parent::__construct() [OK]

Common Mistakes:

• Assuming parent constructor runs before child's code
• Ignoring order of echo statements
• Thinking parent's constructor output appears last