Using a third-party library with PHP

Downloading and Including files

For this example, we’ll use PHPMailer. Grab the latest stable release.

The main file we need is class.phpmailer.php

First we’ll include the file using require_once().

Require_once is similar to include(). The difference between the two is what happens when we encounter an error. If we use the include command and the file doesn’t exist, PHP will throw a warning but it’ll execute the rest of the code. On the other hand, if you use require and the file doesn’t exist then PHP will error and will not execute any more code.

The other difference between them is how many times PHP will include the file. If you use include and require on a file multiple times, it’ll load that file multiple times.

PHPMailer should be included once.

There are actually four commands we might use

  • include()
  • include_once()
  • require()
  • require_once()


Composer is a dependency manager for PHP, like npm is for node.js. The good thing about using Composer is that it makes copying your particular project setup fast and easy, all you need to do is include the composer.json file and anyone can then install the exact same versions of the libs you used on their systems.

Instead of copying all the bulk, (or uploading it to Github), they can just copy one file and re-install it on their computers without wasting bandwidth and time. Once you have the composer.json file, you can include the vendors folder (that is where Composer installs stuff) in .gitignore


Linux / Unix / Mac

curl -sS | sudo php -- --install-dir=/usr/local/bin --filename=composer

Confirm install with: composer -V

Getting started

In your project folder, create the composer.json file with the command

composer init

To add a package to the composer.json file

composer require

If you have an existing composer.json file, install dependencies with

composer install
Using the libraries composer installed in your project

To use the library you installed via composer in your project, all you need is a require statement in your script, like so:

require 'vendor/autoload.php';