Hi fm86, Welcome to WebmasterWorld!
I agree you should probably write a PHP script to accomplish that. Since you mention PHP, I presume you have PHP available on the server; the same deal could be accomplished using C#, Java, ColdFusion, Perl, Ruby... or whatever you have available on the back end.
You'll be using a server-side script anyways, to take the data collected from the <form> and tucking it into an XML format, and into the database? So this name concatenation could easily be accomplished at the same time.
If you have authority to change the XML schema, I'd recommend you store first and last name separately...
You can never tell whether you'll be able to parse apart someone's full name. What if someone enters a first but not a last, or a last but no first. Or they enter two names as their first? as in "F. Scott Fitzgerald". Is the first name "F.", or "F. Scott"?
If you store the data just as you accept it, in separate fields, you will prevent much suffering