PHP表单Action添加$ variable

Is it possible to add in an form action an $ variable ?

I want generate an random number after my url.

script:

<form action="welcome.php" method="post">
Name: <input type="text" name="name"><br>
E-mail: <input type="text" name="email"><br>
<input type="submit">
</form>

On this way its dont work:

<form action=' . $welcome . $urlext' method="post">

Code i want add like this:

$welcome= "welcome.php";

header('Location:' . $welcome . $urlext);

Url looks like this after it: welcome?Ssh4bVY0cKAy

The Problem, i must do it within the form action="" because i have connected all my SESSION in my big script and if i use if button submit then redirect i must change much code.

Hope anyone can help me and thats possible!