php下载mysql数据：报错

php下载mysql数据.量比较大，大约有5到50万的数据。

php从数据库中读取，输出txt给用户下载

$list="连接数据库，查询数据";
$SelectField="这个表的字段";


header ("Content-Type: application/octet-stream");
$filename = "1.txt";
if (preg_match ("/MSIE/",$_SERVER['HTTP_USER_AGENT'])) {
header ('Content-Disposition:  attachment; filename="' . $filename . '"');
} elseif (preg_match ("/Firefox/",$_SERVER['HTTP_USER_AGENT'])) {
header ('Content-Disposition: attachment; filename*="' . $filename . '"');
} else {
header ('Content-Disposition: attachment; filename="' . $filename . '"');
}

foreach ($SelectField as $key => $value) {
     //输出字段              
}

foreach ($List as $keyTop => $valueTop) {
	echo "\r\n";
	foreach ($SelectField as $key => $value) {
			echo $value;	//输出字段里每一个值	
	}
}

数据库和程序不是同一服务器。不可以用sql语句生成txt