In the previous section, we looked at the use of Backbone.js and Underscore.js to identify the usage of open source JavaScript libraries within the WordPress core. Now, it's time to identify the use of open source PHP libraries within the WordPress core, so we will choose PHPMailer among a number of other open source libraries. PHPMailer is one of the most popular e-mail sending libraries used inside many frameworks as a plugin or library. This library eases the complex tasks of creating advanced e-mails with attachments and third-party account authentications.

PHPMailer has been added to Github for improving its development process. You can get more information about this library at https://github.com/PHPMailer/PHPMailer.

WordPress has a copy of this library integrated into the core for all e-mail-related tasks. We can find the PHPMailer library inside the wp-includes folder within the file called class.phpmailer.php.