如何用php文件读取系统变量

<?php
$database_host = getenv('DATABASE_HOST');
$database_user = getenv('DATABASE_USER');
$database_pass = getenv('DATABASE_PASS');

echo "Database Host: " . $database_host . "\n";
echo "Database User: " . $database_user . "\n";
echo "Database Pass: " . $database_pass . "\n";
?>

<?php
$server_name = $_SERVER['SERVER_NAME'];
$document_root = $_SERVER['DOCUMENT_ROOT'];

echo "Server Name: " . $server_name . "\n";
echo "Document Root: " . $document_root . "\n";
?>

<?php
// 设置一个新的配置选项
ini_set('display_errors', 'On');

// 获取一个配置选项的值
$error_reporting = ini_get('error_reporting');

echo "Error Reporting: " . $error_reporting . "\n";
?>

<?php
// 临时设置环境变量
putenv('DATABASE_HOST=localhost');
$database_host = getenv('DATABASE_HOST');

echo "Database Host: " . $database_host . "\n";
?>