Passing index value from select to inside php block in one page html

I have two select dropdown. Example:If i select index 3 in the first select option,the display of the 2nd select option should stop at index 3.What is the best way of doing it.See my current code below.

<label class="control-label">SY1:</label> 
<select class="form-control" name="pri_sy" id="pri_sy" onchange="">
<?php 
    $null = 'Null';
    $choose = 'Choose School Year';
    $Present = 'Present';
    echo '<option value='.$null.'>' .$choose.'</option>';
    echo '<option value='.$Present.'>' .$Present.'</option>';
    for($i=date('Y');  $i > date('Y')-27; $i--){
        $x = $i +1;
        $y = $i;
        echo '<option value='.$y.'-'.$x.'>'.$y.'-'.$x.'</option>';
    }?>
</select>

Result: 
[0]Choose School Year
[1] Present
[2] 2017-2018
[3] 2016-2017   if selected index[3] / How can i pass 3 in my 2nd for loop?
   2015-2016   for($i=date('Y');  $i > date('Y')-3; $i--)/See below php code..
[5]2014-2015
[6]2013-2014
and so on ...



<label class="control-label">SY2:</label> 
    <select class="form-control" name="sec_sy" id="sec_sy" onchange="">
    <?php 
        $null = 'Null';
        $choose = 'Choose School Year';
        $Present = 'Present';
        echo '<option value='.$null.'>' .$choose.'</option>';
        echo '<option value='.$Present.'>' .$Present.'</option>';
        for($i=date('Y');  $i > date('Y')-3; $i--){
            $x = $i +1;
            $y = $i;
            echo '<option value='.$y.'-'.$x.'>'.$y.'-'.$x.'</option>';
        }?>
    </select>

Expected Result:
Choose School Year
Present
2017-2018
2016-2017    
2015-2016



<script type="text/javascript">

$('.form-control').on('change', function(e) {
  var pri_sy = $("#pri_sy").find("option:selected").index();
  var sec_sy   = $("#sec_sy").find("option:selected").index();
  var sen_high_sy = $("#sen_high_sy").find("option:selected").index();
  var college_sy = $("#college_sy").find("option:selected").index();

  var isPri = $(this).is("#pri_sy");
  var isSec = $(this).is("#sec_sy");
  var isSenHigh = $(this).is("#sen_high_sy");
  var isCollege = $(this).is("#college_sy");
  //console.log("from:"+fromIdx,"to:"+toIdx);
  if (isPri) {
    alert('a');
    $('#sec_sy').val(pri_sy);
    alert(pri_sy);
       // $('#sec_sy').each(function(pri_sy, sec_sy) {     
      // $(sec_sy).attr('disabled', pri_sy < sec_sy);
     });
  }  
}).change();
</script>

via Chebli Mohamed