define('DT_ROOT', str_replace("\\", '/', dirname(__FILE__)));
$fp = fopen(DT_ROOT."/test.txt", "w");//文件被清空后再写入
if($fp)
{
    $count=0;
    for($i=1;$i<=5;$i++)
    {
        $flag=fwrite($fp,"行".$i." : "."Hello World!\r\n");
        if(!$flag)
        {
            echo "写入文件失败<br>";
            break;
        }
        $count+=$flag;
    }
    echo "共写入".$count."个字符";
}
else
{
    echo "打开文件失败";
}
fclose($fp);