PHP使用外部命令导出数据库，备份到服务器并下载到本地

<?php    // $dumpFileName目录要有可写权限    $DbHost = 'localhost';　　$DbUser  = 'root';　　$DbPwd   = '123456';　　$DbName  = 'a';   $fileName = $DbName . '_MySQL_data_backup_' . date('YmdHis) . '.sql';　　$dumpFileName= "/var/$fileName";　　header("Content-Disposition: attachment; filename=" . $fileName);　　header("Content-type: application/octet-stream");　　header("PRagma:no-cache"); 　　header("Expires:0");    　　echo `mysqldump -h $DbHost -u$DbUser -p$DbPwd $DbName > $dumpFileName`;    　　$hd = fopen($dumpFileName, 'rb');　　echo fread($hd, filesize($dumpFileName));　　fclose($hd);     ?>