解析PHP对现有搜索引擎的调用

<?php
    $key = $_GET['key'];                        //获得关键字
    $select = $_GET['select'];                  //获得搜索引擎的选择
    switch($select)                             //根据搜索引擎的不同跳转到不同的页面
    {
        case "google":                          //GOOGLE
            $url = "http://www.google.com/search?q=".$key;
            header("Location: $url");
            break;
        case "yahoo":                           //YAHOO
            $url = "http://search.yahoo.com/search?p=".$key;
            header("Location: $url");
            break;
        case "baidu":                           //百度
            $url = "http://www.baidu.com/s?wd=".$key;
            header("Location: $url");
            break;
        default:                                //如果搜索引擎不存在，结束程序
            break;
    }
?>