Constructor inheritance in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constructor inheritance with parent::__construct() call

What is the output of this PHP code when creating an instance of class Child?

PHP

<?php
class ParentClass {
    public function __construct() {
        echo "Parent constructor called. ";
    }
}

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

new Child();
?>

A Child constructor called.

B Parent constructor called. Child constructor called.

C Parent constructor called.

D Fatal error: Call to undefined method parent::__construct()

💡 Hint

Remember that calling parent::__construct() runs the parent's constructor.

The Child constructor calls parent::__construct(), so the parent's constructor message prints first, then the child's message prints.

❓ Predict Output

intermediate

Output when child constructor does not call parent constructor

What will be the output when creating an instance of Child if the child constructor does NOT call parent::__construct()?

PHP

<?php
class ParentClass {
    public function __construct() {
        echo "Parent constructor called.";
    }
}

class Child extends ParentClass {
    public function __construct() {
        echo "Child constructor called.";
    }
}

new Child();
?>

A Child constructor called.

B Parent constructor called. Child constructor called.

C Parent constructor called.

D Fatal error: Cannot call parent constructor

💡 Hint

If the child constructor does not call the parent constructor, the parent's constructor code does not run.

Since the child constructor overrides the parent's and does not call parent::__construct(), only the child's message prints.

🔧 Debug

advanced

Identify the error in constructor inheritance

What error will this PHP code produce when creating an instance of Child?

PHP

<?php
class ParentClass {
    public function __construct($name) {
        echo "Hello, $name";
    }
}

class Child extends ParentClass {
    public function __construct() {
        parent::__construct();
        echo " from Child.";
    }
}

new Child();
?>

A Hello, from Child.

B Child constructor called without error.

C Fatal error: Call to undefined method parent::__construct()

D Fatal error: Uncaught ArgumentCountError: Too few arguments to function ParentClass::__construct()

💡 Hint

Check if the parent constructor requires any arguments and if they are provided.

The parent constructor requires one argument $name, but the child calls it without arguments, causing an ArgumentCountError.

📝 Syntax

advanced

Syntax error in constructor inheritance

Which option contains a syntax error in the constructor inheritance code?

PHP

<?php
class ParentClass {
    public function __construct() {
        echo "Parent";
    }
}

class Child extends ParentClass {
    public function __construct() {
        parent::__construct()
        echo "Child";
    }
}

new Child();
?>

A Missing semicolon after parent::__construct() call

B Missing parentheses in parent::__construct call

C Missing function keyword before __construct

D Missing class keyword before Child

💡 Hint

Check the line where parent::__construct() is called.

The line calling parent::__construct() is missing a semicolon at the end, causing a syntax error.

🚀 Application

expert

Determine the final output with multiple inheritance levels and constructor calls

Given the following PHP code with three classes, what is the output when creating an instance of GrandChild?

PHP

<?php
class Base {
    public function __construct() {
        echo "Base ";
    }
}

class ParentClass extends Base {
    public function __construct() {
        parent::__construct();
        echo "Parent ";
    }
}

class GrandChild extends ParentClass {
    public function __construct() {
        parent::__construct();
        echo "GrandChild";
    }
}

new GrandChild();
?>

A Base GrandChild Parent

B GrandChild Parent Base

C Base Parent GrandChild

D Parent Base GrandChild

💡 Hint

Each constructor calls its parent's constructor first, so the output order follows the inheritance chain from base to child.

The GrandChild constructor calls ParentClass::__construct(), which calls Base::__construct(). So the output prints in order: Base, Parent, GrandChild.

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