PHPMailer is one of the most popular tools for sending emails in PHP. Its popularity stems from its flexibility and ease of use, especially when working with an SMTP server. SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending email messages over the internet. This guide will show you how to set up PHPMailer and use SMTP to send emails.

Step 1: Installing PHPMailer

First, you need to install PHPMailer. This can be done using Composer, a tool for managing dependencies in PHP. If you have Composer installed, simply run:

composer require phpmailer/phpmailer

Step 2: Configuring PHPMailer to Use SMTP

After installing PHPMailer, you can begin configuring it to send emails using SMTP. Here's an example of a simple script:

<?php
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;
use PHPMailer\PHPMailer\Exception;

require 'vendor/autoload.php';

$mail = new PHPMailer(true);

try {
    //Server settings
    $mail->SMTPDebug = SMTP::DEBUG_SERVER;                      // Enable verbose debug output
    $mail->isSMTP();                                            // Send using SMTP
    $mail->Host       = 'smtp.example.com';                     // Set the SMTP server to send through
    $mail->SMTPAuth   = true;                                   // Enable SMTP authentication
    $mail->Username   = This email address is being protected from spambots. You need JavaScript enabled to view it.';               // SMTP username
    $mail->Password   = 'your_password';                        // SMTP password
    $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;         // Enable TLS encryption; `PHPMailer::ENCRYPTION_SMTPS` encouraged
    $mail->Port       = 587;                                    // TCP port to connect to, use 465 for `PHPMailer::ENCRYPTION_SMTPS` above

    //Recipients
    $mail->setFrom(This email address is being protected from spambots. You need JavaScript enabled to view it.', 'Mailer');
    $mail->addAddress(This email address is being protected from spambots. You need JavaScript enabled to view it.', 'Joe User');     // Add a recipient

    // Content
    $mail->isHTML(true);                                        // Set email format to HTML
    $mail->Subject = 'Here is the subject';
    $mail->Body    = 'This is the HTML message body <b>in bold!</b>';
    $mail->AltBody = 'This is the body in plain text for non-HTML mail clients';

    $mail->send();
    echo 'Message has been sent';
} catch (Exception $e) {
    echo "Message could not be sent. Mailer Error: {$mail->ErrorInfo}";
}

Step 3: Security Considerations

When sending emails via SMTP, it's important to keep security in mind. Ensure that:

1. You're using a secure connection: Using encryption, such as TLS or SSL, is essential for securing your email transactions.

2. Login credentials are securely stored: Never store passwords and usernames in your source code. Use secure methods for storing sensitive data, such as through environment variables.

3. Handle errors and exceptions properly: Proper error handling will help you catch and deal with problems related to sending emails.

Step 4: Testing

Before deploying your email solution, it's important to conduct thorough testing. Ensure that emails are delivered to both standard email clients and various webmail services.

Using PHPMailer for sending emails via SMTP is an efficient and flexible solution for many PHP applications. With proper configuration and security measures, you can ensure reliable and secure email sending for your business or personal projects.