use .innerHTML in a echo statement in a php file

hello

this is two file i used
------------------------
testform.php
<form method="post" action="doForm" >
user name <input type="text" name="userName" id="userNameID" />
<span id="userConfirm"></span>
<input type="submit" value="Submit">
---------------------------

doForm.php
if the user name doesn't exist, then i use the following two line:
--------------------------------------------------
if ( username doesn't exist) {
include("testform.php");
        echo "<script language=Javascript>
        document.getElementById('userConfirm').innerHTML='User Name doesn't exist.';
        </script>";
}
----------------------------------------------------
the javascript doesn't work. seems like there is some error in the js file.
any suggestion about this?

Thanks very much,
lilyyan