Path problem?

1 

class_xml.trigger();
comboFunction = new Object();
comboFunction.change = function (vt){
  trace(vt.target.selectedItem.label);
  
  _root.student_mc.student_xml.direction = "receive";
  
  _root.student_mc.student_xml.URL = "student.php?course="_root.course_mc.course_cb.selectedItem.label;
  _root.student_mc.student_xml.URL = "student.php?classes="+vt.target.selectedItem.label;
  _root.student_mc.student_xml.trigger();
  _root.sub.text=vt.target.selectedItem.label;    
  
}
class_cb.addEventListener("change", comboFunction);
stop();

Error

Error Symbol=class_mc, layer=Layer 1, frame=1:Line 9: Syntax error.
_root.student_mc.student_xml.URL = “student.php?course=”_root.course_mc.course_cb.selectedItem.label;

what is the problem??

my php code

<?php 
//this line includes the database connection variables
include_once("config.php"); 
$key = $HTTP_GET_VARS[course];
$keya = $HTTP_GET_VARS[classes];
$charset=mysql_query("SET NAMES utf8;"); 
$charset=mysql_query("SET CHARACTER_SET_CLIENT=utf8;"); 
$charset=mysql_query("SET CHARACTER_SET_RESULTS=utf8;"); 
$result = mysql_query("SELECT studentNO, courseName, studentClass,studentChname FROM student WHERE studentClass LIKE '%".$keya."%' AND courseName LIKE '%".$key."%'"); 
// And now we need to output an XML document 
// We use the names of columns as <row> properties. 
echo '  <?xml version="1.0" encoding="UTF-8" ?> ';
echo '<datapacket>'; 
while($row=mysql_fetch_array($result)){ 
    $line = '<row studentNO="'.$row[studentNO].'" studentChname="'.$row[studentChname].'"/>'; 
    echo $line; 
} 
echo '</datapacket>'; 
?>

is there any problem on $key = $HTTP_GET_VARS[course];
$keya = $HTTP_GET_VARS[classes]; ??