jQuery动态追加选择BOX来调用PHP的mysql函数

Question

How to call PHP Function From jQuery

getstudent('"+$("#classroom").val()+"')

I sure MySQL had 4 records about classroomid='c1'

But I can't show 4 records to selectbox student

In HTML

<select  name="classroom" id="classroom" class="required"><option value="c1" selected>Room-1</option><option value="c2">Room-2</option></select>

In jQuery

$("#selectstudent").append("<tr><td><select name='student["+x+"]' id='student"+x+"'><?=getstudent('"+$("#classroom").val()+"')?></select>")</td></tr>").trigger('create');

In PHP

function getstudent($roomid){
  $query = "Select studentid,studentname From student Where classroomid='$roomid' Order By studentid";
  $sql=mysql_query($query);
  if (mysql_num_rows($sql)){
    while ($rs = mysql_fetch_assoc($sql)){
      $str = $str."<option value='".$rs["studentid"]."'>".$rs["studentname"]."</option>";
    }
  }else{
    $str="";
  }
  //test//
  $str = $str."<option value=''>".$query."</option>";
  $str = $str."<option value=''>".mysql_num_rows($sql)."</option>";
  return $str;
}

I only got "Select studentid,studentname From student Where classroomid='c1' Order By studentid" And "0" in selectbox student

If I delete "Where classroomid='$roomid'" from $query，It can show all data into selectbox student

I paste "Select studentid,studentname From student Where classroomid='c1' Order By studentid" to querywindow of phpMyAdmin and run，It's show 4 records