Traits in OOP in PHP

As you already know, in PHP, you cannot inherit from multiple classes at once, only from one. We have previously covered a solution to this problem: instead of inheritance, use objects of some classes inside others.

PHP has another way. It consists of using traits. A trait represents a set of properties and methods that can be included in another class. In this case, the properties and methods of the trait will be perceived by the class as its own.

The syntax of a trait is the same as that of a class, except that the trait name needs to be declared using the keyword trait.

You cannot create an instance of a trait - traits are intended only for connection to other classes. The connection itself is carried out using the command use, after which the name of the trait to be connected is specified, separated by a space. This command is written at the beginning of the class.

Let's look at the application of traits with a practical example. Suppose we are given this trait Helper, containing private properties name and age, as well as their getters:

<?php
	trait Helper
	{
		private $name;
		private $age;
		
		public function getName()
		{
			return $this->name;
		}
		
		public function getAge()
		{
			return $this->age;
		}
	}
?>

Suppose we also have this class User, in whose constructor the properties name and age are set:

<?php
	class User
	{
		public function __construct($name, $age)
		{
			$this->name = $name;
			$this->age = $age;
		}
	}
?>

Let's now add getters for the properties of our User class. But we won't write them in the class itself, we'll just connect the Helper trait, which already has these methods implemented:

<?php
	class User
	{
		use Helper; // connect the trait
		
		public function __construct($name, $age)
		{
			$this->name = $name;
			$this->age = $age;
		}
	}
?>

After connecting the trait, the methods and properties of this trait will appear in our class. We will access them as if they were methods and properties of the class itself:

<?php
	$user = new User('john', 30);
	echo $user->getName(); // will output 'john'
	echo $user->getAge();  // will output 30
?>

Implement the City class with properties name, age, population and getters for them. Let our class use the trait Helper we have already created to shorten its code.