PHP 函数在开发中的作用是什么？

<?php
function connect_db() {
  $servername = "localhost";
  $username = "username";
  $password = "password";
  $dbname = "database";

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

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

  return $conn;
}

// 使用连接函数
$conn = connect_db();

// 执行 SQL 查询
$result = $conn->query("SELECT * FROM users");

// 输出查询结果
while ($row = $result->fetch_assoc()) {
  echo $row["id"] . " " . $row["name"] . "<br>";
}

// 关闭连接
$conn->close();