Echoing PHP Variable inside XML

[Cracky]

I'm collecting form information and I have to post that data to a URL in XML format. So I'm putting the XML into a post string and using cURL to execute the post. How would I echo the proper form fields into the XML? In the ProjectId I put what I thought would be the code, but Notepad++ isn't showing it as being valid PHP so I think that's wrong. Thanks!\[code\]$post_string = '<?xml version="1.0" encoding="UTF-8" ?><Ping vid="" sid=""> <ProjectInfo> <ProjectId><? echo $projectid; ?></ProjectId> <SubProject></SubProject> </ProjectInfo> </Ping> ';\[/code\]I was using this link to figure it out: http://www.codediesel.com/php/posting-xml-from-php/