mysql实现自增字符串

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(255) NOT NULL,
    user_id VARCHAR(255) NOT NULL UNIQUE
);

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 插入数据
$username = "JohnDoe";
$sql = "INSERT INTO users (username) VALUES (?)";
$stmt = $conn->prepare($sql);
$stmt->bind_param("s", $username);

if ($stmt->execute()) {
    $last_id = $stmt->insert_id; // 获取自增ID
    $user_id = "user_" . str_pad($last_id, 4, '0', STR_PAD_LEFT); // 生成自增字符串
    $update_sql = "UPDATE users SET user_id = ? WHERE id = ?";
    $update_stmt = $conn->prepare($update_sql);
    $update_stmt->bind_param("si", $user_id, $last_id);
    $update_stmt->execute();
    echo "新记录插入成功，用户ID为: " . $user_id;
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$stmt->close();
$conn->close();
?>