Constructor Declaration in Interface in OOP in PHP

In an interface, you can also declare a class constructor. Let's look at an example. Suppose we decided to make a Rectangle class, which will have methods for finding the area, perimeter, and also a constructor that takes two parameters. Let's describe our class using an interface:

<?php
	interface iRectangle
	{
		public function __construct($a, $b); // constructor with two parameters
		public function getSquare();
		public function getPerimeter();
	}
?>

Let's write the implementation of our interface iRectangle:

<?php
	class Rectangle implements iRectangle
	{
		private $a;
		private $b;
		
		public function __construct($a, $b)
		{
			$this->a = $a;
			$this->b = $b;
		}
		
		public function getSquare()
		{
			return $this->a * $this->b;
		}
		
		public function getPerimeter()
		{
			return 2 * ($this->a + $this->b);
		}
	}
?>

What did declaring a constructor in the interface give us? Firstly, we will not forget to implement the constructor in the class. Secondly, the interface explicitly indicates that the class constructor must accept two parameters: no more, no less. This also protects us from accidental error.

Why did we create a separate interface iRectangle, and did not add the constructor to the Figure interface? Because all figures have a different number of sides and, accordingly, a different number of parameters in the constructor. Therefore, we had to create a separate, more precise interface specifically for rectangles.

Make an interface iCube, which will describe the Cube figure. Let your interface describe a constructor that takes the side of the cube as a parameter, as well as methods for getting the volume of the cube and the surface area.

Make a Cube class that implements the interface iCube.

Make an interface iUser, which will describe a user. It is assumed that the user will have a name and age and these fields will be passed by the constructor parameters. Let your interface also specify that the user will have getters (but not setters) for the name and age.

Make a User class that implements the interface iUser.