要使用PHP發送HTML格式的電子郵件，您可以使用PHPMailer這樣的庫。以下是使用PHPMailer發送HTML郵件的步驟：

1. 首先，確保已經安裝了PHPMailer庫。如果還沒有安裝，可以使用composer進行安裝：

composer require phpmailer/phpmailer

2. 創建一個新的PHP文件，例如send_html_email.php，并在文件中包含autoload文件以及初始化PHPMailer類：

<?php
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;
require 'vendor/autoload.php';

$mail = new PHPMailer(true);

3. 配置SMTP服務器設置：

try {
    //Server settings
    $mail->SMTPDebug = 2;               // Enable verbose debug output
    $mail->isSMTP();                    // Send using SMTP
    $mail->Host       = 'smtp_host';     // Set the SMTP server to send through
    $mail->SMTPAuth   = true;          // Enable SMTP authentication
    $mail->Username   = 'your_username'; // SMTP username
    $mail->Password   = 'your_password'; // SMTP password
    $mail->SMTPSecure = 'tls';        // Enable implicit TLS encryption
    $mail->Port       = 587;            // TCP port to connect to; use 465 if you have set `SMTPSecure = phpseclib::ENCRYPTION_STARTTLS`

4. 設置發件人、收件人以及郵件內容：

    //Recipients
    $mail->setFrom('your_email@example.com', 'Mailer');
    $mail->addAddress('recipient@example.com', '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';

5. 發送郵件并檢查結果：

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

6. 完整的代碼示例：

<?php
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;
require 'vendor/autoload.php';

$mail = new PHPMailer(true);

try {
    //Server settings
    $mail->SMTPDebug = 2;               // Enable verbose debug output
    $mail->isSMTP();                    // Send using SMTP
    $mail->Host       = 'smtp_host';     // Set the SMTP server to send through
    $mail->SMTPAuth   = true;          // Enable SMTP authentication
    $mail->Username   = 'your_username'; // SMTP username
    $mail->Password   = 'your_password'; // SMTP password
    $mail->SMTPSecure = 'tls';        // Enable implicit TLS encryption
    $mail->Port       = 587;            // TCP port to connect to; use 465 if you have set `SMTPSecure = phpseclib::ENCRYPTION_STARTTLS`

    //Recipients
    $mail->setFrom('your_email@example.com', 'Mailer');
    $mail->addAddress('recipient@example.com', '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}";
}
?>

將上述代碼中的smtp_host、your_username、your_password、your_email@example.com、recipient@example.com等值替換為您自己的SMTP服務器設置和電子郵件地址。運行此PHP腳本，如果一切配置正確，收件人應該會收到一封包含HTML內容的電子郵件。