PHP:如何获取一个表单请求,该表单请求嵌套有其他参数想要发送的iframe?

I have a form like the flowing:

<form name="pdf" method="post" action="pdf.php">
Name:<input type="text" name="command" id="command" size=20 placeholder="command name">
...
...
<iframe name='rpc' width=700 height=330 frameborder=0 allow-forms>unimportant options that you may forget easilly</iframe>
<input type="submit" name="submit" value="submit">
</form>

In this iframe there will contain the flowing html code:

<textarea name="other" cols=650 rows=20 id="other"></textarea> 

So how could I get the value of the param other(the value of the textarea) in the server side?

It's null when I try to use $_POST["other"];