Constructor inheritance in PHP - Step-by-Step Execution

Concept Flow - Constructor inheritance

Define Parent class with __construct

↓

Define Child class

↓

Child inherits Parent's __construct?

↓

Child calls Parent::__construct

↓

Create Child object

↓

Parent __construct runs

Shows how a child class can inherit or override the parent's constructor and how object creation triggers these constructors.

Execution Sample

PHP

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

class ChildClass extends ParentClass {
}

new ChildClass();

This code shows a child class inheriting the parent's constructor and calling it when a child object is created.

Execution Table

Step	Action	Evaluation	Output
1	Define ParentClass with __construct	ParentClass::__construct exists
2	Define ChildClass extends ParentClass	ChildClass inherits ParentClass
3	Create new ChildClass object	ChildClass has no __construct, inherits ParentClass::__construct
4	Call ParentClass::__construct via ChildClass object	ParentClass::__construct runs	Parent constructor called
5	End of script	No more code to run

💡 Script ends after constructor output

Variable Tracker

Variable	Start	After Object Creation	Final
ChildClass object	undefined	object created	object exists

Key Moments - 2 Insights

Why does the parent's constructor run when creating a child object if the child has no constructor?

Because the child class inherits the parent's constructor automatically if it does not define its own, as shown in step 3 and 4 of the execution_table.

What happens if the child class defines its own constructor?

The child's constructor overrides the parent's, so only the child's constructor runs unless it explicitly calls parent::__construct(), which is not shown in this example.

Visual Quiz - 3 Questions

Look at the execution table, what is output at step 4?

A Child constructor called

B Parent constructor called

C No output

D Error

💡 Hint

Check the Output column at step 4 in the execution_table

At which step is the ChildClass object created?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the Action column for object creation in the execution_table

If ChildClass had its own __construct method, what would happen when creating a ChildClass object?

A Child constructor runs instead of parent

B Parent constructor runs automatically

C Both constructors run automatically

D No constructor runs

💡 Hint

Refer to key_moments about overriding constructors

Concept Snapshot

Constructor inheritance in PHP:
- Child class inherits parent's __construct if none defined
- Creating child object calls parent's constructor if inherited
- Child can override __construct to customize
- To run both, child must call parent::__construct() explicitly

Full Transcript

This example shows how constructor inheritance works in PHP. The ParentClass defines a __construct method that prints a message. The ChildClass extends ParentClass but does not define its own constructor. When we create a new ChildClass object, PHP uses the parent's constructor automatically. The execution table traces these steps: defining classes, creating the object, and running the constructor. The variable tracker shows the child object being created. Key moments clarify why the parent's constructor runs and what happens if the child defines its own constructor. The quiz tests understanding of output, object creation step, and constructor overriding behavior.