PHP语法（3.）环境变量

<!--PHP的环境变量 -->
<?php
echo "环境变量\n";
echo "浏览器的版本:"."$_SERVER['HTTP_USER_AGENT']"."\n";
echo "客户端的IP地址:".$_SERVER['REMOTE_ADDR']."\n";
echo "网站的主机名:".$SERVER['HTTP_HOST']."\n";
echo "客户端的端口:".$SERVER['REMOTE_PORT']."\n";
echo "网站所有的目录:".$SERVER['DOCUMENT_ROOT']."\n";
echo "服务器的操作系统:".$SERVER['OS']."\n";
echo "服务器的系统路径:".$SERVER['PATH']."\n";
?>
<!--PHP的系统常量 -->
<?php
echo "当前服务器操作系统:".PHP_OS."\n";
echo "当前PHP的版本:".PHP_VERSION."\n";
echo "当前的行号:"._LINE_."\n";
echo "当前的PHP文件:"._FILE_."\n\n";
$s=2;
echo "圆的半径:".$s."\n";
echo "圆的周长:".M_PI*2*$s."\n";
?>