PHPMailer

PHPMailer is a long-running PHP library for composing and sending email, supporting SMTP (with authentication and TLS/SSL), the local sendmail binary, or PHP’s built-in mail() function as interchangeable transports. It handles the parts of email that are easy to get wrong by hand: MIME multipart message construction, attachments (including inline images), HTML email with automatic plain-text alternative generation, character-set and encoding handling, and DKIM signing.

The core classes (PHPMailer, SMTP, POP3, Exception, OAuth) live under src/ with no required external dependencies for the base library, though optional Composer packages add OAuth2 authentication (Gmail, Outlook) and other SMTP providers. It has been a foundational dependency for a large share of the PHP ecosystem — including WordPress’s core mail functions — for over two decades.

What You Get

• A unified PHPMailer class supporting SMTP (with auth and TLS/STARTTLS), local sendmail, or PHP’s mail() as interchangeable send methods
• Automatic MIME multipart message construction, including file attachments, inline/embedded images, and HTML email with an auto-generated plain-text alternative
• Built-in SMTP and POP3 classes for lower-level protocol interaction when the high-level API isn’t enough
• OAuth2 authentication support (via the companion OAuth class) for Gmail, Outlook, and other providers requiring modern auth instead of plain SMTP credentials
• Multi-language error message support (language/ directory) covering dozens of locales
• DKIM signing support for improving email deliverability and reducing spam-folder placement

Common Use Cases

• Sending transactional emails (password resets, order confirmations, notifications) from a PHP application via a real SMTP relay instead of PHP’s unreliable mail()
• Sending HTML marketing or newsletter-style emails with inline images and file attachments from a self-hosted PHP app
• Authenticating against Gmail/Outlook SMTP with OAuth2 instead of storing a plaintext app password
• Powering the email layer of a CMS or framework plugin (PHPMailer is the library WordPress core itself uses for outgoing mail)

Under The Hood

Architecture - The library is organized as a small set of focused classes under src/: PHPMailer.php is the main facade handling message composition and orchestrating the send; SMTP.php implements the SMTP protocol directly (commands, response parsing, TLS negotiation) rather than delegating to a third-party SMTP client; POP3.php supports POP-before-SMTP authentication for legacy mail servers; OAuth.php/OAuthTokenProvider.php implement OAuth2 token handling for providers like Gmail and Outlook; and DSNConfigurator.php parses connection-string style configuration. This is a deliberately dependency-light design — the base library needs no external packages to send mail. Tech Stack - Pure PHP with a Composer manifest (composer.json) defining the package as a library type; PHPCS (phpcs.xml.dist) enforces coding standards, PHPUnit (phpunit.xml.dist) drives the test suite, and phpDocumentor (phpdoc.dist.xml) generates API docs. get_oauth_token.php is a standalone helper script for obtaining OAuth2 refresh tokens interactively. Code Quality - The test/ directory contains a substantial PHPUnit suite (PreSendTestCase.php plus dedicated fixtures for PHPMailer, SMTP, POP3, OAuth, and language tests), including fake SMTP/POP3 server shell scripts (fakesendmail.sh, fakepopserver.sh) so tests can exercise real protocol exchanges without a live mail server. The project maintains a SECURITY.md and UPGRADING.md, reflecting the operational discipline expected of a library embedded in millions of production codebases including WordPress core. API Design - Sending a basic email is a handful of property assignments ($mail->Host, $mail->addAddress(), $mail->Subject, $mail->Body) followed by a single $mail->send() call, with sensible defaults for encoding and MIME boundaries; the API predates PHP’s modern typed-property conventions but remains one of the most widely documented and copy-pasted email APIs in the PHP ecosystem.