With the development of the Internet, email has become one of the important means of daily communication for people. In Web applications, sending emails has become one of the necessary functions, and with the development of Web applications, the way of sending emails has also been greatly improved. Among them, the email sending method based on the SMTP protocol has become one of the most commonly used ways to send emails in Web applications. Next, this article will introduce in detail the complete process of sending emails using PHP SMTP protocol.

1. Introduction to SMTP protocol

SMTP (Simple Mail Transfer Protocol) is a simple mail transfer protocol, which is a protocol used to transfer mail between computers. When sending emails, the SMTP protocol needs to be transmitted through multiple servers. Therefore, when sending emails, you need to specify the server that sends the email and the server that receives the email. The SMTP protocol has two link modes: plaintext mode and encryption mode. Plain text mode refers to the SMTP protocol that does not encrypt. The plain text of data transmission can be intercepted and tampered with by malicious users. Encryption mode uses the SMTP protocol to establish a secure connection to ensure data transmission security.

2. The process of sending emails using PHP using SMTP protocol

The process of sending emails is roughly divided into three steps: establishing connection, authenticating identity, and sending emails.

1. Establishing a connection

Before using the SMTP protocol, you need to determine the address and port number of the SMTP server. Generally, the address of the SMTP server can be obtained from the email service provider. The port number of the SMTP server is generally 25 or 465. When using the SSL security protocol, the port number is 587. After determining the SMTP server address and port number, you need to establish a connection with the SMTP server through PHP code.

The PHP code to establish the connection is as follows:

<?php
$smtp_server = ''; // SMTP服務(wù)器地址
$smtp_port = ''; // SMTP服務(wù)器端口
$smtp_socket = fsockopen($smtp_server, $smtp_port, $errno, $errstr, 30);
?>

The fsockopen function is used to establish a connection with the SMTP server. If the connection is successful, the $smtp_socket variable will contain the connection to the SMTP server. Otherwise, the $errno and $errstr variables will contain error information about the connection failure.

2. Authentication Identity

The SMTP protocol requires that the identity must be authenticated before sending emails. Commonly used SMTP identity authentication methods include PLAIN, LOGIN, and CRAM-MD5. Among them, the PLAIN authentication method does not encrypt the password, while the other two methods will encrypt the password.

The PHP code for SMTP identity authentication is as follows:

<?php
$email_address = ''; // 發(fā)送方的Email地址
$email_password = ''; // 發(fā)送方的Email密碼
if ($smtp_socket && $email_address && $email_password) {
    // 進行身份認證
    fwrite($smtp_socket, "AUTH LOGIN
");
    fwrite($smtp_socket, base64_encode($email_address) . "
");
    fwrite($smtp_socket, base64_encode($email_password) . "
");
}
?>

Among them, the base64_encode function is used to encode the email address and password to prevent interception and password theft.

3. Send email

After completing SMTP identity authentication, you can send emails through the SMTP protocol. Before sending an email, you need to set the email content, subject, recipient, sender and other information. The email content can be written in html code or in plain text format.

The PHP code for sending emails using the SMTP protocol is as follows:

<?php
$to = ''; // 收件人Email地址
$subject = ''; // 郵件主題
$content = ''; // 郵件內(nèi)容
$email_from = ''; // 發(fā)送方Email地址
$email_to = ''; // 收件人Email地址
if ($smtp_socket && $to && $subject && $content && $email_from && $email_to) {
    // 設(shè)置郵件頭部
    fwrite($smtp_socket, "MAIL FROM:<{$email_from}>
");
    fwrite($smtp_socket, "RCPT TO:<{$email_to}>
");
    fwrite($smtp_socket, "DATA
");
    // 設(shè)置郵件主體
    fwrite($smtp_socket, "From: {$email_from}
");
    fwrite($smtp_socket, "To: {$to}
");
    fwrite($smtp_socket, "Subject: {$subject}
");
    fwrite($smtp_socket, "Content-Type: text/html; charset=utf-8
");
    fwrite($smtp_socket, "
");
    fwrite($smtp_socket, "{$content}
.
");
    // 結(jié)束SMTP會話
    fwrite($smtp_socket, "QUIT
");
}
?>

In the above code, the email header and body are set, and the email content is sent through the SMTP protocol. If the email is sent successfully, information indicating that the email was sent successfully will be returned.

3. Summary

This article briefly introduces the complete process of sending emails using PHP SMTP protocol. Among them, establishing a connection, authenticating identity, and sending emails are key links in the entire process. By studying this article, you will master the entire process of sending emails using the SMTP protocol, and understand how to establish a secure connection and perform SMTP identity authentication. At the same time, you can also set the email header and body according to actual needs to achieve customized email sending functions.

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