PHP
Constructor and abstract
Learn to define a constructor in an abstract class and call it with parent:: in a subclass.
Constructors in abstract classes
You can define a __construct inside an abstract class. This constructor cannot be called directly with new (because the class is abstract); but it runs when a subclass is instantiated with new. So the constructor provides shared initialization for subclasses.
For example, the Vehicle abstract class assigns the $name property in its constructor. Subclasses do not rewrite this logic; they inherit it.
If the subclass does not define its own constructor, the parent's is used automatically. But if the subclass writes its own constructor, it must call parent::__construct() explicitly.
Call the parent with parent::__construct
If the subclass defines its own constructor, the parent's constructor is not called automatically. You must call it explicitly with parent::__construct(). Otherwise the parent's initialization logic is skipped.
The typical pattern: the subclass constructor takes extra parameters, first calls parent::__construct(), then assigns its own extra properties.
class Car extends Vehicle {
public int $wheels;
public function __construct(string $name, int $wheels) {
parent::__construct($name);
$this->wheels = $wheels;
}
}
This pattern preserves shared initialization while adding subclass-specific fields.
Exercise Abstract constructor + parent call
Define a Vehicle abstract class with a public string $name property. Its constructor public function __construct(string $name) should assign $this->name = $name. Then derive Car from Vehicle. First declare public int $wheels in Car. The Car constructor public function __construct(string $name, int $wheels) should first call parent::__construct($name), then assign $this->wheels = $wheels. Finally, a describe() method should do return $this->name . " " . $this->wheels . " wheels";. Use $c = new Car("Toyota", 4) and echo $c->describe() . "\n";
Show example solution
:__construct($name), then $this->wheels = $wheels. describe() returns $this->name . " " . $this->wheels . " wheels".
Example solution:
<?php
// Define the Vehicle abstract class
abstract class Vehicle {
public string $name;
public function __construct(string $name) {
$this->name = $name;
}
}
class Car extends Vehicle {
public int $wheels;
public function __construct(string $name, int $wheels) {
parent::__construct($name);
$this->wheels = $wheels;
}
public function describe(): string {
return $this->name . " " . $this->wheels . " wheels";
}
}
$c = new Car("Toyota", 4);
echo $c->describe() . "\n";
Test yourself
-
If you define a constructor in the abstract class, what must the subclass do?
- Nothing; the parent constructor is called automatically
- Call parent::__construct() explicitly
- Cannot redefine the constructor
- The subclass cannot define a constructor
Show answer
Call parent::__construct() explicitly — Correct. When the subclass defines its own constructor, it must call parent::__construct() explicitly.
-
Parent constructor call
public function __construct(string $name, int $wheels) {
____
$this->wheels = $wheels;
}
Show answer
public function __construct(string $name, int $wheels) {
parent::__construct($name);
$this->wheels = $wheels;
}
-
Match the constructor and abstract concepts
- abstract constructor
- Defined in abstract class
- parent::__construct
- Calls the parent constructor
- shared init
- Common setup step
- subclass constructor
- Can take extra parameters
Next lesson: Summary - Abstract classes
