<?php
// Object Arguments
require_once("Car.php");
$subaru = new Car("2019", "Subaru", "WRX STi");

<?php
require_once('Vehicle.php');

class Engine {
  //...
}

class Transmission {
  //...
}

class Car extends Vehicle
{
  //...
}

If we wanted to extend the Engine class, we can’t do something like this:

class Car extends Vehicle, Engine { … }

Your editor will automatically highlight the Engine and say that it is incorrect syntax. So what can we do? First let’s add a method to the Engine class and change the class to a Trait since the Engine is technically a trait of a car.

<?php
trait Engine {

    public function check_oil_level() {
        echo "Engine oil level good";
    }
}

Let’s do the same for our Transmission Trait.

<?php
trait Transmission {

    public function check_oil_level() {
        echo "Transmission oil level good";
    }
}

Both the Engine and the Transmission traits have the same method name, check_oil_level(). The Engine Trait method returns “Engine oil level good” and the Transmission Trait method returns “Transmission oil level good.”

Great, how do we use the traits within our Car class now? With the use keyword. Let’s build up on this like we normally do and start simple, by implementing the Transmission trait and checking the transmission oil level.

<?php
//...
require_once("Transmission.php");

class Car extends Vehicle {
  
  use Transmission;
  
  //...
}

$subaru = new Car("2019", "Subaru", "WRX STi");
$subaru->check_oil_level();

We should get the following response: “Transmission oil level good.”

We can still call methods from our Vehicle parent class by calling the method name like normal.

echo $subaru->getYear(); // example from Parent class

Let’s move to the next step by adding our Engine trait to the Car class.

We get the following error: Trait method ‘check_oil_level’ will not be applied, because it collides with ‘Transmission.’ If you still wanted to give it a shot, refresh your page and you’ll get the following error.

How do we get around this? We have to tell PHP which check_oil_level() method we want to use. How can we do that? With special syntax. Once you use both of the traits, add curly braces and use the insteadof keyword. I want to use this method instead of this other method.

<?php
//...
require_once("Transmission.php");
require_once("Engine.php");

class Car extends Vehicle {
  
  use Transmission, Engine {
    Transmission::check_oil_level insteadof Engine;
  }
    
  //...
}

$subaru = new Car("2019", "Subaru", "WRX STi");
$subaru->check_oil_level();

Now when we call the check_oil_level(), PHP knows which method to call. Great, that solves one problem. But what if we wanted to check our engine oil as well? You’ll just have to rename one of them within the Car class. Our Transmission check_oil_level() will be used as the default. Our Engine oil check will be named check_engine_oil_level().

Calling our methods now yields the proper results. That’s it when it comes to Traits. If you start receiving errors, it’s most likely due to method name collisions, which you now know how to redefine.