PHP mail()如何设置发件人邮件

这是我的代码：

$to = '[e-mail]';

$subject = 'test';

$body = 'test';

$header  = 'MIME-Version: 1.0' . "\r\n";
$header .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
$header .= "To: <$to>" . "\r\n";
$header .= 'From: [e-mail] \r\n';

mail($to,$subject,$body,$header);

代码有效,它会发送电子邮件.
但发件人不是我定义的那个.
发件人似乎是网络邮件主机.
我究竟做错了什么？

尝试设置信封发件人,并在邮件标题中设置发件人,如下所示：

$to = "[e-mail]";
$from = "[e-mail]";
$subject = "subject";
$message = "this is the message body";

$headers = "From: $from"; 
$ok = @mail($to,$message,$headers,"-f " . $from);