PHP OOP

Let's say you had a class that you wanted to use, but there was some sort of error in creating the object. This might be that the wrong parameters were passed, or the third party service (eg. a database) wasn't available at the time of creation.

If this happens you'll obviously want to handle the error correctly, but the question is would you throw an exception in the constructor or handle the error condition elsewhere?

Note that this article will talk about PHP, but the same discussion applies to other languages that implement OOP principles.

The PHP Standards Recommendations (called PSR) are a set of standards that aim to make certain aspects of working with PHP easier. They include things like coding standards (PSR-1), sending HTTP requests (PSR-7), and the autoloading standard PSR-4.

PSR-4 describes the ability to include PHP classes based on their file paths. This means that instead of manually including every class file you need, you can write an autoloader that will do this for you.

The key bits of information are the namespace and the name of the class. Using this information you can then look up and include the class you need as the code is running.

Reflection is used to gather more information about code during the runtime of the program. This allows code to inspect itself and to make small modifications, which is useful in a variety of situations.

This is possible in PHP thanks to the reflection classes, which are built into core. PHP is capable of using reflection to inspect all types of classes, object, functions, variables and even the recent addition of Fibers. Using the reflection classes we can find out the internal structure of a class that an object is part of, including properties, methods constants as well as what sort of class we are dealing with. Reflection works even if the property of method is private or protected.

This post will concentrate on objects (and their associated classes) as the code used to inspect objects can generally be applied to other forms in PHP.

The instanceof operator in PHP is great at making sure you are looking at a type of object before acting on it. Whilst it's straightforward to use on its own, using it can lead to some undesirable side effects.

As a basic example, let's create a couple of interfaces and classes to show the instanceof operator in action. Rather than just have some test names I am using object names that you might find in a system. This consists of User and Order that might form part of a commerce system.

interface UserInterface {}
class User implements UserInterface {}

interface OrderInterface {}
class Order implements OrderInterface {}

If we instantiate the User object we can detect if the user is an instance of the class User like this.

A Data Transfer Object (DTO) is a way of taking data out of a database or API and present it in a unified way across your application. As a design pattern this has uses in standardising how a particular bit of data is passed around, without having to resort to using arrays to accomplish the same job.

I was doing some work with PHP's PDO library the other day when I noticed that I could fetch data out of a database using the fetchAll() method with the PDO::FETCH_CLASS flag. This flag automatically returns the data in an object. This got me thinking more about how this worked, but I couldn't find many articles talking about this. Hence this post.

Setting Things Up

Let's take a class called SpecialText that will store an ID and some text, along with a __toString() method to allow the printing of those values. We will use this class throughout this post to store data from the database.

The other day I was conducting a code review and found that a developer had used a trait to give two classes the same group of utility methods. Whilst there was nothing wrong with this in terms of functionality, I asked the developer why they had chosen to use traits instead of inheritance. We eventually decided that an inheritance model would be better suited to the situation but I thought I would go through some of the thought processes here.

What Is A Trait?

A trait, if you weren't aware, is like a class, but you don't instantiate it directly. Traits are defined using the trait keyword and are otherwise quite like a class in structure.

The idea is that code is essentially copied into the class you want to use it in from the trait and the class acts like it had that code all along. For example, let's take a simple trait.

SOLID is a set of object oriented design principles aimed at making code more maintainable and flexible. They were coined by Robert "Uncle Bob" Martin in the year 2000 in his paper Design Principles and Design Patterns. The SOLID principles apply to any object oriented language, but I'm going to concentrate on what they mean in a PHP application in this post.