$idList = array();
$nameList = array();
foreach(array_keys($course) as $k)
{
	$idList[$k['course_id']] = 1;
	$nameList[$k['course_id']] = $course[$k['course_name']];
}
$txt = "<select name='whatever'>";
foreach(array_keys($idList) as $id)
{
	$txt .= "<option value='{$id}'>{$nameList[$id]}</option>\n";
}
$txt .= "</select>";