php redirect header conditional querystring

(I am a beginner in php)

I can set up a redirect that works.

<?php
header("Location: http://www.MyWebsite.org/SomePage.php");
## more code here 
?>

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...but what I want to do is to redirect based on conditions in the code
and
fill-in the URL's querystring variables:

<?php
require_once "Mail.php";
include_once "MyConnect.php";

## create and assign default values to 3 variables:
$successful=FALSE;
$redirt=99;
$redirm=99;

## processing code here
## variables would be the querystring values

if(blablabla)
	{
	$successful=FALSE;
	$redirt=42;
	$redirm=31;
	}
else
	{
	if(something)
		{
		$successful=TRUE;
		$redirt=4;
		$redirm=2;
		}
	else
		{
		$successful=TRUE;
		$redirt=28;
		$redirm=13;
		}
   }

## ok, now, use the variables in the querystring AND use the correct header redirect destination:

if(!$successful)
{
header("Location: http://www.MyWebsite.org/SignUpPage.php?paratitle=".$redirt."&msgbody=".$redirm."");
exit();
}
else
{
header("Location: http://www.MyWebsite.org/MessagePage?paratitle=".$redirt."&msgbody=".$redirm."");
exit();
}

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but I am getting the dreaded error:
"Warning: Cannot modify header information - headers already sent by..."

I understand that I cannot send anything to a browser before calling the "header" function. What I think that means in that I cannot use "echo" anywhere on the page. Or maybe I can have an echo on the page, but if it is invoked, the redirect will not work and I'll get the error.

My page is a processing page, all php, no HTML.  I know that MyConnect.php is strictly php, no HTML, but I am not sure about the Mail.php file (maybe that is the problem.)

So, is this doable? If yes, is my coding problem obvious?

Thanks!

Dennis